Painting welded aluminum boats

[kmorin]

I've had some PM's, some personal emails and a couple of recent (local) conversations about painting aluminum boats. We've discussed this in many past threads but I decided to summarize my opinions on the subject in a single thread so it could be a point of view for anyone who's not read the AAB.com Forum for a long time. 

This thread is about painting marine grade aluminum alloy metal, especially that material fabricated, by welding, into an aluminum boat. Aluminum alloys are labeled by a numbering system that identifies the range of alloying metals added to pure aluminum to gain specific mechanical and chemical properties where “marine grade” alloys are a few among hundreds of aluminum alloys. 

I’ll discuss painting 5000 and 6000 series aluminum only. More exactly, I’ll address painting 5052, 5083 & 5086 sheet/plate and 6061 series extrusions as these represent the greatest percentage of welded aluminum boats in the under 40’ LOA range we see in the US market.  (Embossed sheet, or tread plate will behave similarly if it is 60 series aluminum but I have not painted any 3000 series diamond tread so I can't say the methods suggested will apply to those alloys.) 

As with any online opinion article, the statements here are my opinions based on designing, building, then painting aluminum boats of the alloys listed, and what those paint jobs has taught me. I may have concluded in error (?) and would be happy to have correction of the points I make that are not accurate. The sole purpose of this thread is to help summarize a series of steps that will lead to a good quality paint job on welded aluminum that can be followed by owners, builders or restorers of these boats. 

A point to separate out of this “How To” discussion is ‘shooting paint’. I never learned to apply paint at an automotive level of finish- I refer to this as “shooting paint” or applying paint. I’ve always employed someone from the auto body painting skilled trade to shoot the best paint jobs I’ve done. The few boats I painted myself always ended up with a run, drip or orange peel.... or even fish-eyes and other defects. So, I absolutely respect anyone that has taken time to learn to shoot paint well; but I will be concentrating on preparation, not applying paint. 

I view the skill in applying the paint film separate from the discussion about adherence to the metal or paint prep and the associated results.

I have also inspected numerous paint jobs by others, most unknown in origin, that have succeeded or failed to remain intact in different time frames and fully adhered to the underlying metal.  My experience with painting aluminum boats and observing others' paint jobs has lead me to believe as facts; what I list below. After providing the ‘facts’ I’ll try to explain how these conditions contribute to success or failure of paint films on marine grade aluminum.

1. Aluminum alloys, like pure aluminum, form an oxide film (aluminum oxide), with in 3-4 seconds of exposure to the atmosphere. Aluminum oxide forms and is chemically bonded to the underlying metal at a molecular level. 

2. Aluminum oxide formation is the primary reason the metal has commercial uses by mankind. Aluminum oxide is a self-healing film that stops metallic aluminum from losing molecules to outside attractants which cause corrosion. Unlike rust, or Ferric Oxide, aluminum oxide film stops the chemical decomposition of the underlying metal; rust continues until the steel/iron is completely consumed as rust flakes where aluminum remains intact as long as its oxide film remains intact. 

3. 5000 series alloy sheets and plates are produced on rolls that add a shiny film called mill scale. This film is not the same as Aluminum Oxide. Mill scale is not water vapor proof, and will hold and retain moisture from the atmosphere. Mill scale is not chemically well-attached to the underlying metal, it is mechanically adhered during the roll forming of sheet/plate. 

4. Paint films can adhere to aluminum chemically or mechanically or by both means. Chemical adhesion is stronger and more lasting than mechanical adhesion but both can anchor or adhere paint film coatings for various time periods and provide different levels of long term adhesion. 

5. Chemical adhesion begins by removing both layers normally found on new 50series aluminum. Both the mill scale and aluminum oxide need to be replaced by another chemically bonded film that will better ‘hold’ paint films as aluminum oxide will not hold paint films as long, nor as well, as other chemically bonded oxides. 

6. Removing mill scale and aluminum oxide can be done with fairly strong acids, bases, or by mechanical means. Any acid with a ph below 4 or base above 10 will remove mill scale and aluminum oxide but the solutions that are farther toward 1 for acids and 14 for bases will prove more effective removal in shorter time. 

7. Etching or chemical removal of mill scale and aluminum oxide still allows reformation of aluminum oxide in 3-4 seconds IF the etched metal surface is allowed exposure to the atmosphere before any other chemical agents are applied.  Oxide reforms on bare metal nearly instantly.

8. Mechanical removal of the mill scale and oxide can be done at anytime an abrasive tool or stream of particles is applied to aluminum with sufficient force to ‘scrape’/cut/excavate the two films off the parent metal. Aluminum oxide will reform within 3-4 seconds of the ‘sanded/blasted’ surface being exposed to the atmosphere as oxygen will combine with pure aluminum alloy to form the aluminum oxide in that time frame. 

9. Mechanical adhesion preparation occurs by abrading the sheet surface. Abrasion can be accomplished by ‘sand blasting’ (semi-hard particles at high speed impacting metal surface) or by rotary or belt sanding methods. The goal of mechanical abrasion is to create an “Anchor Pattern” on the metal surface which can be thought of like ‘Velcro (TM) hooks’ or like ‘tiny hairs’ when viewed at a microscopic level. 

10. Paint films can hold/cling/adhere to the anchor pattern’s texture protrusions and ‘valleys’ by solidifying around the roughened surface like casting epoxy around hair fibers or concrete curing around re-bar or added fibers. This method of adhesion is not the same as chemical adhesion since it relies on a mechanical hardening of the film (solidification, catalyzation, ‘curing’) around the ‘filaments and crevices’ instead of molecular level bonding as a film to the substrate. 

11. Some surface treatments of aluminum can provide a micro-sized anchor pattern film developed by the nature of the chemical that is used to treat and bond to the underlying aluminum. These ‘oxide replacement’ (perhaps ‘oxide substitution’) chemicals provide a bond to the underlying metal at a chemical level while providing paint films an exposed surface consisting of a mechanical anchor pattern created by the dried chemical structure of the film-causing agent. 

IF these facts provided aren’t understood, or if you have a different understanding of the facts regarding the surface nature of marine aluminum, then; the next post(s) won’t make as much sense. The steps to a good paint job are explained in ongoing posts but they rely on these facts as foundational to those steps. 

If we need to detail more clearly or correct my assertions(?) I’d welcome any ideas about my conceptions of the physical facts. Next I’ll layout the methods I think are correct for painting aluminum boats.  I'd especially appreciate it if Forum member Chaps could review and remark as he has more aluminum boat painting experience that I have, by far.

Cheers,
Kevin Morin
Kenai, AK             
 
 

[kmorin]

This post provides a general outline of the steps in painting welded aluminum boats and some explanation of the first two steps.

1. If sand blasting or sanding will be employed as preparation (?); abrade the surface of the metal and clean. 

2. Tape-off or ‘skirt’ any area not being etched or painted. 

3. Etch the surface to be painted with acid solution and rinse with water Keeping Metal Wet. 

4. Apply Chromic Acid (Alodine/Chem Film or other similar product) to the water covered (still wet) metal to convert the aluminum surface film to Chrome Oxide instead of Aluminum Oxide. 

5. Allow alodine/chromic oxide film to dry on metal.

6. Repeat 3 & 4 for spots of blue or green color indicated as not converted in #4. 

7. Apply primer. 

8. Apply paint. 

These steps take into account the facts presented in the previous post and taken in sequence will result in the best performing paint film on aluminum boats. Some painters only take steps 1, 2, then 7 & 8. This system relies on a mechanical adhesion to the ‘anchor pattern’ but some primers may add some of step 3 and maybe even 4 as well? 

I have seen a very mixed bag of results from ‘self-etching’ primers. I have seen paint jobs that began to slough off in a single season, and others with similar products last for years without problems. I don’t have any personal experience with self-etching primers as I’ve always used acid etch and Alodine conversion when I’ve had control of a paint job. 

Some coating methods rely solely on step #4 as the main step in preparing the metal for primer and some industrial parts are only treated as in #4 and left with a chrome oxide based film as the finish although I’m not familiar with this method as applied to boats. The following points are about the steps listed above. 

1. Creating an anchor pattern on aluminum by ‘sand blasting’ can be done with various abrasive materials. The hardness and sharpness of the particles combined with the pressure of the air and distance to the metal being blasted all effect how the metal will be cleaned and what anchor pattern is created.

In general, softer media are used on aluminum when compared to steel. Hard sandblasting abrasives are garnet sand, steel slag, even aluminum oxide particles. These media types can actually excavate or gouge out the surface of aluminum instead of just cleaning and leaving an anchor pattern. For this reason softer media is most often used on aluminum including walnut shells, white silica sand or even ‘baking soda’ which is used on clad aluminum sheeting of aircraft. 

It is accepted that blast media leave the most effective anchor pattern on aluminum but rotary or belt sanding will also create an anchor pattern and many paint systems are applied over a ‘buffed’ or ‘sanded’ surface preparation. Sanding pads of 80 to 120 grit work fine to create an anchor pattern. Much finer or coarser grits tend to either polish the metal or to gouge and leave and uneven surface but I’m sure others’ experiences with sanding to obtain a paint film’s anchor pattern may differ? 

Also, 3M’s Scothbrite (TM) brown or coarse abrasive pads will also create an anchor pattern using either rotary or belt sanders to drive this abrasive over the metal. I have a 50” drum sander that has a hook and loop strip of replaceable abrasive which can be used to clean the mill scale (and temporarily oxide film) from sheet aluminum. I have also done this process with a hand held belt sanders using Scotchbrite and various sized rotary or random orbital sanders to clean aluminum of mill scale. 

Aluminum need not be painted to last a long time. Simply removing the mill scale and allowing the naturally forming oxide film to form will protect aluminum from corrosion unless specific conditions are present that induce or promote corrosion. Mill scale can be left on aluminum, however, the appearance results in mottled coloration and ‘white flowers’ (beginning corrosion cell sites) isn’t nearly as attractive as buffed or etched hulls. 

Cleaning after blasting or sanding consists of dust removal using shop air or a water hose rinse. Either blasting or buffing should remove any grease or oil that was on the metal. If not; any smears of oil and grease, including hand prints or glove prints, can be removed using acetone, MEK, or xylene all of which require prudent PPE selection and use. 

Both sand blasting and sanding by power tool will require a long time even for a small skiff. If you’re doing this work yourself make sure to budget a realistic time to do all the work implied. IF you’re hiring the work done plan to pay a fortune as the cost will be based on time and materials along with cleaning the site if your sand blast.

Painting only takes a few hours total to shoot/apply paint but the prep stages will take much (MUCH) longer. This is one of the reasons many owners prefer to leave their boats unpainted. 

2. Speaking of time allotted to prepare for painting; taping-off or draping and skirting areas that won’t be painted or will be painted with different types or colors is also time consuming. Blue or green painter’s (paper masking) tape will work to hold thin plastic sheet materials to cover areas from being painted or from over spray and paint dust. It's common to see a drape of plastic film around the waterline boot when painting the topsides to cover the bottom from the different type of paint and to see that film/skirt lifted to cover the topsides when the bottom is painted. Covering hatches, windows, companionways and other parts of a hull with a film of thin plastic adhered with painter’s tape is common practice. 

Lots of deck fittings and many deck or gunwale mounted hardware items won’t be painted- like cleats, deck fill fittings and various other hardware. If any item won’t be painted and it can be dismounted from the hull by removing bolts or screws then a good paint job will see these items removed, taped off and added in to the overall paint system.

Items welded on but not painted will need to be ‘taped off’. Taping-off or covering parts of a welded boat to avoid paint includes wrapping an item with painter’s tape or wrapping aluminum foil around that item (like handrails or tie-too railing) and taping the ends of the foil covered areas with a tape ring to seal. Aluminum foil applies pretty rapidly by simply folding it or hand wrapping around pipes, hand rails and other odd shapes that will be excluded from the paint scheme. 

Next, I’ll try to present some information regarding acid etching in preparation for other coatings. 

Cheers,
Kevin Morin
Kenai, AK
 
 
 

[kmorin]

Thanks largely to the highway tanker truck support industry, several chemical companies offer aluminum cleaning and ‘brightening’ chemical solution products. Most contain one or more acids which, when applied to aluminum, will remove the entire surface film by chemical reaction, we refer to this process as ‘etching’ aluminum. 

Not to be confused with etching for pattern imprints on aluminum, this process is used to remove both mill scale and aluminum oxide film in a wide area. Sprayed or sponged on application is intended to create a uniformly colored and cleaned surface with minimal effort added after applying.

However, if your boat has lots of old mill scale, flowering or stains and splashes of various materials it may be necessary to use a deck brush and abrade the surface after you apply the acid and while it is foaming (reaction that strips surface film of aluminum) to get a clean white metal etch? 

I prefer a product called Zep-a-Lume from the Zep Chemical company but I understand that Zep isn’t available in all states as various regulations prohibit transport or handling due the acid content of the solution. I won’t try to list the many other products that could be available in this category of chemicals, except to note; they should all be considered industrial grade, aggressive acid solutions and treated with care and not casually. 

Handling, preparing, then applying and diluting these materials requires Personal Protective Equipment (PPE) to be correctly selected and faithfully worn for safety and health as well as environmental concerns. I suggest that all a full body suit, like a foul weather oil skins (rain suit) and head covering, be used. Rubber boots, slicker pants, rain coat and a hat of some type should be worn when spraying etching acid. A face shield should be worn OVER splash-proof goggles, and disposable gloves should be worn underneath elbow length ‘parts washer gloves’ with the cuffs turned down to prevent dripping into the outer gloves. 

Splash-proof goggles have a layer of foam - like a weather strip - around the edges where the goggle fits to the face and avoid any droplets of acid from ‘bouncing’ off the face into the eyes. A grinding shield type of face protection is to insure splashes, droplets or mists carried by the breeze; don’t get in contact with your face and the splash goggles insure a second layer of protection for the eyes. 

A half face, rubber sealing, air purifying respirator is required to keep from breathing the mist or fumes when acid is applied. The correct filtration cartridges are Organic Vapor cartridges (charcoal) with mist/dust filter covers as pre-filters and if you’re not familiar with these - please don’t think a piece of paper and rubber band are a valid substitute! 

The warmer the acid, regardless of concentration, and warmer the metal; the cleaner and more uniform the etched surface will be left. If you live where it’s not above 70 degrees most of the year (like here in Alaska) then heating the acid with warm water and using a propane weed burner to warm the boat prior to spraying are important steps-unless you etch inside the shop. 

Most acid solution's containers will have some written comment about dilution or the amount of water that should/could be added to the full strength for common applications. In general, the stronger the acid concentration the more reactivity and the shorter time it will take to lift the mill scale and oxide films and the more thoroughly the surface will be cleaned. 

In some cases, working out doors, we’ve had to redo the etch three or more times to get a fully white metal etch as the cooler temperatures reduced the reactivity of the mix we used. Test patches are the best way to understand what your acid solution (level of concentration versus dilution) will do to the metal you’re cleaning. 

If the test patch shows a vigorous foaming action when the acid is applied and left to react.... and when you rinse with the water hose after the foaming action slows down shows a cleanly uniform white colored aluminum is left? Then things are working well. However, if the acid is too dilute, metal or acid cold, and the boat is not newly built so the old mill scale has additional corrosion films on the surface.... you may be left with streaks or unevenly cleaned and striped coloration in the etched areas. 

In order to make sure you don’t have uneven coloration and vertical strips is make sure you ALWAYS begin at the lowest point of the metal to be etched and work upward. By spraying horizontally in 3’ wide patches and overlapping slightly as you move up each spray pass; the cleaning action will be more uniform and no streaks will form unless the metal is cold where incomplete etching occurs. In any case, by repeating the acid application to any area; eventually you can clean to uniform white metal etch. 

Once the etching solution has foamed/reacted with the metal the ‘suds’ of foam will begin to collapse and it's time to begin rinsing continuously with water. A water hose works fine, there is no temperature concern to speak of as the primary acid cleaning reaction has passed and now the concerns are to keep the metal wet in order to exclude atmospheric oxygen from chemically combining to form aluminum oxide and to dilute the acid & metal oxide solution to neutral ph. 

As a rule of thumb, continuous spraying with water for 5 minutes will usually rinse all the diluted acid off and provide sufficient volume of water mix in the runoff that even a “ph strip” shows a neutral ph. 

The reason to etch metal that has an anchor pattern imprinted onto it is to create a surface where a chemical bonding agent can be applied which can improve paint adhesion beyond just the mechanical adhesion of paint films drying around ‘tiny hairs’ raised as the anchor pattern. 

Next, to get the chrome oxide film bonded to the now cleaned and oxide free aluminum. 

Cheers,
Kevin Morin
Kenai, AK
 
 

[kmorin]

This post about converting aluminum oxide to chrome oxide represents my description of something about which I have limited chemical knowledge. I am not a chemical engineer or very knowledgeable, overall, about physical chemistry. Therefore, if we had the good fortune to have someone with real in-detail knowledge of the subject who would be willing to comment here (?) it would be great to hear from them. 

Aluminum is non-porous and smooth when finished on sheet rolls as our 5000 series marine alloys are. Paint won’t stick very well unless there are enhancements to that surface making it more “paint friendly.” We’ve reviewed, above, the chemical and mechanical methods to get paint to adhere to welded metal boats’ materials. 

The chemical method involves converting a thin layer of aluminum and oxygen (aluminum oxide) over to another film that will provide a better anchor for paint(s). If aluminum has been etched then the aluminum oxide layer is removed and if it's then covered with water to exclude atmosphere; aluminum oxide is prevented from forming. At this point another acid (alodine or chromic acid) can be introduce to the surface and that acid will chemically bond with the metal’s molecules. This second acid is chromic acid or Alodine.

Alodine can be sprayed, rolled on or sponged onto wet aluminum and spread as evenly as possible. When this happens the acid reaction will combine chrome molecules with aluminum molecules so a layer of chrome oxide is formed instead of aluminum oxide. All that is needed is to wear proper PPE (discussed previously) and apply the Alodine solution and let it dry in place. 

When dry, the surface will range form light to dark brown, golden to dull dirt colored. If there are patches of blue or green then the conversion hasn’t happened at a molecularly bonded level - and those patches need to be re-etched with Zep, and then rinsed and finally have Alodine re-applied and dried. 

The color indication is in areas that didn’t bond, and so there is no chrome oxide intact to which primer could adhere. 

I have worked on jobs that needed to be converted where I couldn’t effectively spray the alodine so I used a sponge but that wasn’t effective ENOUGH so; I resorted to paper towels. In this instance I wetted paper towels by soaking them in a plastic trough with alodine and then laid the towels on the etched and wetted metal and left them for a few minutes. One of the guys in that project laid water dampened towels on the wing surface and they used a spray bottle to add alodine to the wetted towel.  That too, worked well to keep the aluminum wet while waiting to get the alodine on before the aluminum dried and formed its own oxide.

This method, while kind of extreme allowed the most control of the alodine and least mess by over-spray or drips even though the embossed pattern of the toweling actually showed in the pattern of converted metal surface. The resulting paint job didn’t print this pattern through even the primer; so, while an odd method, it worked to convert the metal where spraying or sponging on the chrome conversion acid wasn’t acceptable. 

Alodine is toxic if ingested, I understand from the bottle labels, so care is needed in handling and using this chemical. I have seen it marketed under the name “aluminum conversion” fluid or some similar wording. 

Alodine’s dried surface, if we were to look at it microscopically, is “rough” compared to the same surface of the underlying aluminum. In essence, chrome oxide has a very small microscopic anchor pattern when it dries due to the crystal structure of the dried fluid. It is this rough texture that primer paints can anchor to or adhere to and it is this property that makes the efforts to convert the surface from aluminum oxide to chromic oxide worth the effort. 

Once the process gets to point #7. (first post) I don’t have much more to add as the painter, the manufacturer’s representative and their product information should take over as a guide.  I've tried to explain the most complete method of PREPARING to apply paint, not to explore the many paint brands with the numerous product lines any or all of which have their own 'take' on aluminum preparation.

If you have two fishermen, or two boat builders, or two welders, two painters, two electricians or chemical vendors’ representatives.... in a conversation..... you will, as the boat owner, builder, or remodeler: HAVE three or four opinions of how to take any given step in the list I’ve posted.  Those opinions will also include... "He's nuts- doesn't know what he's talking about"  Typically, someone with a different opinion is basing those kinds of remarks on their aluminum working experiences.  I am basing my remarks on my experiences too. 

Opinions always seems to differ from person to person, and the number of various opinions always exceeds the number of people in the discussion by at least twice. (3X?) In some cases I’ve seen consensus that the sun was up or down, but even then; its rare to find agreement on the need for any given step in painting welded aluminum boats that I’ve listed. 

The next few posts will be attempts to link images that correlate to the information provided above.

Cheers,
Kevin Morin
Kenai, AK
 
 

[kmorin]

Here are couple of images from a boat that seems to have an odd surface treatment?  At least I'm not able to make complete sense of the photo evidence I see in these images, given what I've posted above about hull sheet facts.

First is an overview image of the hull's topside at a paint line.  The area below the blue paint boundary appears to have been etched; but not before the upper area was painted over millscale?



This image shows paint having simply chipped off, fallen off, let go of the hull. This is an image of new boat (1st season as I was told) by a name builder on the West Coast of the US.  Notice the very streaked, mottle gray coloration of the area below the paint line? This type of coloration shows the acid 1. wasn't strong enough? Too dilute? 2. wasn't warm enough?  too cold when applied to cold metal?  3. wasn't left to react to the surface long enough, leaving traces of mill scale that still could retain water vapor.


Zooming in the center of the paint chips you can see a couple of things. First there was/is a large dime-sized area of pitting that was under the paint fostered by the intact millscale which show as a white patch in the center of this image.  The areas to the side of that deeper corrosion show intact millscale and some smaller pitting beginning above the former paint line. 

If paint isn't chemically bonded to the metal via Chrome Oxide or very well mechanically bonded in a deep anchor pattern; the paint film will not adhere for long, and under the paint corrosion will be promoted compared to bare aluminum open to the weather.

Also, some pitting and flowers show in the area of the hull below the paint line, indicating the millscale wasn't completely removed or the mill scale removal attempt happened some time after the boat was in use?  I wasn't able to find anyone to provide a better understanding.



Last image of this paint chipping on a skiff's topsides shows the millscale remains intact on the majority of the area where paint just released.  I see a red tinted layer in some areas along the edges of the blue paint line, but I'm not sure if this was a red primer or a previous paint layer? What seems evident (?) is that the paint didn't adhere to the millscale very well.

At the lower edge of the paint line, the etch seems to reach upward a very small fraction of an inch?  This might indicate the topsides etch wasn't done after the painting, or was done before and the millscale coated areas above the eventual paint line were left intact intentionally?  Can't tell by me.

What I think this shows clearly is that millscale won't hold paint very well, and removing it is good practice- whether done mechanically or chemically?

Next image is of the lower topsides of another name brand skiff- nearly new.


I don't know if this skiff has a clear coating over the millscale (?) but I can see the millscale wasn't fully/successfully removed, or... this skiff may be clear of millscale (?) but the clear topcoat has held water vapor and created a series of white flower corrosion sites?

Below is an image of net skiff build (and etched) in the 1980's.  There are no white flowers, no corrosion (plenty of mud and fish entrails) and this skiff will not corrode due to millscale or pitting anytime soon. 



From bow, looking aft along the port side of a Cook Inlet net skiff- all MIG welded, hollow ribs (5086), bow tank at the sheer, tie-too rail ending on inner false topsides (air tank & chest working surface). 

These images are to help visualize the impact of leaving millscale (and aluminum oxide) intact before painting. However, even if the hull will not be painted, acid etching will remove the millscale and allow a uniform aluminum oxide film to form and preserve a welded aluminum boat.

Cheers,
Kevin Morin
Kenai, AK