Blasting VS Sanding For New Paint

General boating discussion
ehsvp
Posts: 73
Joined: Wed May 05, 2010 6:13 am
13

Blasting VS Sanding For New Paint

#1

Post by ehsvp »

Hi everyone,
I've extensively researched painting an aluminum boat in these forums and in other boat sites on the internet. There is a lot of information out there and more than one way to get satisfactory results. This boat is a home-build which for me limits some of the options for good paint preparation. I've decided that sanding or blasting will be the method I'm going to use for preparing the aluminum for paint. The vast majority of the exterior of the boat will not have any paint at all, I'm mostly concerned with painting the interior of the boat and the rear inside deck area. My concern with sanding all the inside of the hull, stringers etc. is that it will take so long to complete that some of the areas where I started will already be oxidized significantly compared to the areas where I finished. The boat is a 28' Alaskan fishing style boat with a bridge and the interior will be top coated with Zolatone. The cabin is 10' long. I can rent a large tow behind compressor and industrial sandblasting equipment which could get the job done well. I'd prefer to sand the interior if I could. How have any of you that have painted aluminum boats dealt with this problem?

Bob
kmorin
Donator 08, 09, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24
Posts: 1735
Joined: Mon Aug 18, 2008 1:37 am
15
Location: Kenai, Alaska

Re: Blasting VS Sanding For New Paint

#2

Post by kmorin »

ehsvp,
I've painted a few boats of various sizes both inside and outside so I'll start with a generality and try to close in on some specifics.

I'd say sand blasting is the best prep for paint for several reasons. #1 it gets into the corners, nooks and close spaces where sanding disks are held out of reach by their diameter not conforming to the close in spaces. #2 the sanded anchor pattern is not usually as vertical when looked at in a microscopic cross section- so- the sand blast prepped metal is able to offer primer a better 'grip' on the surface. #3 regardless of sanding or blasting some form of oxide removal is needed so the metal has A) a chromium oxide layer or B) some other non-aluminum oxide layer for the primer to bond to SO blasting more thoroughly removes mill scale (not to be confused with aluminum oxide) and allows a better chemical removal of the remaining oxide also contributing to better final bond. (NOTE; some primers are offered that have etching action as part of their application adhesion process- addressing the oxide layer without separate acid wash process)

The blast versus rotary sanding or buffing an area that is open (like a deck) is one thing but as soon as you move inside a cabin area- less and less area can be cleaned/prepped/reached by those tools. So if you're sure you need an anchor pattern inside the cabin (MOST painters don't require any more than an etch inside) then; blasting is about the only way to reach into various furnishings and structural elements found there?

Now, about blasting in an enclosed space! Not for the faint of heart or ill prepared. I've only done it in my ill spent youth (long past) and in each case I used a supplied air full body suit. Now-days those supplied air systems have to conform to some pretty heavy regulatory conditions! The filtering package may be for rent (?) but it might not be inexpensive due to the maintenance on those breathing air units?

If you plan to blast inside the cabin- I'd suggest you research the set up carefully. The other aspect is the need for a partner/tender and for good planning. Last, with blasting inside is the removal/clean up of the media from the bilges! As with the gear to do this work and the overall need to plan - removal and cleaning up is no small task.

Next, to a little experience. I've coated half dozen clients boats' interiors from open skiffs to small cabin cruisers/day boats and did not blast any of the interiors. The acid etch preceded a primer then Zolatone and clear coat. Last I looked those 20-30 year old paint jobs are still doing fine - without any more than acid and allodyne sub primer prep.

I was involved in reconditioning a larger commercial boat and it was sand blasted inside - in prep for an full paint system - and that job was 'roomy' but still took serious efforts to set up, blast, clean up- then paint beginning with the etch and allodyne conversion.

Outside hulls; I've blasted only one hull for paint and sanded quite a few- and left others just acid stripped and then painted. All the paint jobs have stayed on- BUTTTT..... at the rub rail impact area -only the sand blasted paint prep has stood up to dock rash and denting or anchors and fishing gear being hauled and hitting the sides. There are chips in the 'just etched' surface as there are in the sanded or buffed prep. But the sand blasted wear areas seem to hold up the best.

In regard to self etching primer- I have never used it- but several friends have. One says it worked fine, has held up well and he'd use it in the future. Another one removed the entire paint job at his own expense and redid the paint using an etch/allodyne/primer sequence - so he's not so fond of self-etching primer. So I've no real advice there-hopefully others will post more information?

The cost of blasting and painting that boat; approached the cost of the metal work!!!

Hope that helps your exploration and planning? and I hope Chaps will post in reply to your question as he's the Forum's "Pro from Dover" about paint on welded aluminum boats.

Cheers,
Kevin Morin
Kenai, AK
kmorin
Chaps
Donator '09
Posts: 2246
Joined: Sun Jan 13, 2008 12:19 am
16
Location: Seattle, WA

Re: Blasting VS Sanding For New Paint

#3

Post by Chaps »

Thanks Kevin but it should also be added that I'm a "one trick pony" in that field of expertise because my prep processes for painting aluminum include either sand blasting or . . . sand blasting. I get away with that because I only do boat bottoms and in that case all the sand ends up on my nice smooth concrete floor where it is easily swept up. Blasting the interior of a small boat is a different animal and besides being practically impossible to do its also unnecessary in my opinion. In fact I can't imagine why anyone would want to paint the inner hull and framing . . . coatings in there just introduce a maintenance headache IMO. As far as cabin interior prep and using chemical etch primers make sure the aluminum is really clean as the acids in them don't do well when applied over greasy fingerprints, dirt, grime, dried soda pop, etc. I also would encourage using a small variety of electric sanders including a detail machine to scuff everything up well then do your priming. Aluminum sands easily just be sure to wipe it all down again with acetone before applying products. Find a manufacturer like Pettit that has a complete priming/coating system designed to work together. Bail me out here Kevin, my palms are getting sweaty thinking about having to use anything other than sand at 90 psi and epoxy . . .
1987 24' LaConner pilothouse workboat, 225 Suzuki
Image
please view and like: https://www.facebook.com/bottompainting/
ehsvp
Posts: 73
Joined: Wed May 05, 2010 6:13 am
13

Re: Blasting VS Sanding For New Paint

#4

Post by ehsvp »

Dear Kevin & Chaps,
Thanks for the replies. Painting inside the cabin would be above the sole and the bilge would be completely sealed off from blasting material. I don't necessarily want to paint the hull sides, stringers and ceiling framing but my research indicates it should be painted prior to insulating. If I don't have to paint these areas my interior work to be painted that is visible would be minimal. I really would like to acid etch before painting but I don't have a place to drain the rinse water.

Bob
Chaps
Donator '09
Posts: 2246
Joined: Sun Jan 13, 2008 12:19 am
16
Location: Seattle, WA

Re: Blasting VS Sanding For New Paint

#5

Post by Chaps »

I would not do acid wash/alodine, etc for interior work, that is some nasty stuff. Clean the metal, sand it, clean it again with hot thinner, apply Pettit etch primer then tie coat primer then Zolatone. Pettit has a kit http://www.pettitpaint.com/product.asp?id=82 they sell it as an underwater kit but can be used topside as well. You didn't say how you are insulating but can't imagine paint would be required for areas that stay dry even if you are foaming.
1987 24' LaConner pilothouse workboat, 225 Suzuki
Image
please view and like: https://www.facebook.com/bottompainting/
ehsvp
Posts: 73
Joined: Wed May 05, 2010 6:13 am
13

Re: Blasting VS Sanding For New Paint

#6

Post by ehsvp »

Chaps, thank you very much for that information. I was really struggling on deciding on a paint system for the interior. I have one more question about the sanding, If it takes more than one day to sand the interior of the boat could I still apply the etch primer or would I have to resand surfaces that have been exposed for 12 or more hours? I'm totally open to any kind of insulation for the inside of the boat that wouldn't require painting of the inside of the hull or the cabin ceiling framing. Do you have any recommendations for a insulation that could be installed by a home builder?

Bob
Chaps
Donator '09
Posts: 2246
Joined: Sun Jan 13, 2008 12:19 am
16
Location: Seattle, WA

Re: Blasting VS Sanding For New Paint

#7

Post by Chaps »

Personally I think the ultra short fuse meme on getting primers onto freshly abraded aluminum is overblown. I would pay more attention to cleanliness, thoroughness in sanding, meeting temp & humidity reqmts, application technique, mix ratios, etc., stuff that will have more bearing on the success of the paint job than beating an arbitrary oxidation time window. Can't help much on insulation, thin spun glass batts? Why not?
1987 24' LaConner pilothouse workboat, 225 Suzuki
Image
please view and like: https://www.facebook.com/bottompainting/
kmorin
Donator 08, 09, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24
Posts: 1735
Joined: Mon Aug 18, 2008 1:37 am
15
Location: Kenai, Alaska

Re: Blasting VS Sanding For New Paint

#8

Post by kmorin »

Bob, Chaps, in regard oxidation reforming after sanding- total elapsed time to form oxide is about 3 to 4 seconds. That is why the alodine has to be put on while the acid rinse is wet-(when the surface will be converted to chrome oxide) so there is no chance to form aluminum oxide.

There is no difference to sanding and waiting one hour or two years in regard the sanding process' contribution to oxide removal. Same with all other processes that strip aluminum oxide. But sanding does provide a better anchor pattern for certain.

Cheers,
Kevin Morin
kmorin
ehsvp
Posts: 73
Joined: Wed May 05, 2010 6:13 am
13

Re: Blasting VS Sanding For New Paint

#9

Post by ehsvp »

Thanks Kevin, what do you like to use for cabin insulation? Do you paint the interior before installing insulation?

Bob
kmorin
Donator 08, 09, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24
Posts: 1735
Joined: Mon Aug 18, 2008 1:37 am
15
Location: Kenai, Alaska

Re: Blasting VS Sanding For New Paint

#10

Post by kmorin »

Bob, I've only used board/rigid foams or sprayed in foam. Except for one engine box that had to be super quiet- there we used some sound deadening material that was designed for marine sound dampening and acted as insulation too.

I've glued in sheet foam between framing structures/bars/angles in both the sides of cabins, floors/soles/deck of cabins, and sides under the window band. Since these steps are usually covered with from paneling/sheeting/stripping/ceiling materials for finish the insulated areas are sealed for air flow. Also the surface is coated with adhesive- troweled on in our case like linoleum glue- so there wasn't much risk of mill scale vapor retention starting crevice corrosion cell sites- so we did not etch.

I have done a few boats where we etched and sanded off the mill scale as we built- and others where Zolatone's adhesion was in question we etched to get a more assured paint job. But those were not insulated boats. I have not painted under insulation, in our work it was either or. And if I did paint under the insulation I'd only go as far as acid etch and alodine and Maybe(?) stop with a zinc chromate primer. Since the insulation and whatever finish cover it will be over that area- moisture laden air isn't a likely condition so painting beyond prep/primer doesn't seem necessary.

I don't mind spray foam for certain uses, but it's kind of pain to have to work on the boat after its applied! If urethane products are used- it should be where the surface will stay dry as it does saturate and retain water- and WILL promote the formation of corrosion cell sites which I have seen in voids in skiffs, and bilge sections in larger boats.

Spray foam is a labor saver compared to fitting, cutting and gluing in sheet foam/insulation and that is why it is used in many jobs- at least as near as I see? But with good prep- taping, blocking off openings and good clean up- that is a 2nd man following the foam operator and scraping the flat surfaces- spray foam goes in fast and does a great job of insulating.

Hope that more of the guys here will post as I think several have either built of completed/modified their own boats. Maybe some has a better perspective and will be willing to post?

Cheers,
Kevin Morin
Kenai, AK
kmorin
ehsvp
Posts: 73
Joined: Wed May 05, 2010 6:13 am
13

Re: Blasting VS Sanding For New Paint

#11

Post by ehsvp »

Thanks Kevin, good information.

Bob
Post Reply Previous topicNext topic