Scotch-Brite Aluminum Hull

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sirrox
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Joined: Tue May 03, 2016 12:41 pm

Scotch-Brite Aluminum Hull

#1

Post by sirrox » Tue May 31, 2016 1:57 pm

I have a 2006 Weldcraft Maverick 201DV. Some road salt on the hull from a late November towing caused some light cosmetic corrosion (flowering and small white spots). A pressure washer, hot water/soap etc. won't touch them.

CORROSION 1

CORROSION 2

I thought I would try a Scotch-Brite pad (Ultra Fine Grey - 07448) in an inconspicuous spot on the hull. I read a post in another forum that turned out really well using this technique going fore-aft by hand. As you can I sanded the side of my outboard bracket. It turned out beautiful IMO. A very nice brushed looked. The look was exactly what I was after. I didn't take a before of the side I sanded so the before below is the other side of the bracket but they were identical.

BEFORE

AFTER

Of course, I did not expect it to stay exactly perfect like this once it began to oxidize again but it did still look the same 6 days later.... until I dropped it in the salt (Southeast Alaska) for 2 nights. Upon taking it out the area above water line was in decent shape. Some very small calcium like rings which didn't come off with plain soap and water but I was quite OK with the look. The bottom looked very smooth and blemish free but stained as you can see. The water was very dirty/cloudy in the area we were. The rest of my hull was not affected. I did not try try to take the staining off with anything aside of soap and water, and as I mentioned it did nothing.

STAINING

So I just gave the bracket 2 minutes of attention with the Scotch-Brite again. Back to square one.

NOW

So... Questions;
1. How long does it take for aluminum to establish an oxidized layer that will protect itself like the rest of my aged hull?
2. If I put a coating over it (sharkhide, wax, aluminum polish etc.) it will likley stay looking better but I expect it will become a maintenance issue as the aluminum will be protected from oxygen and will not oxidize correct? I'll need to keep on the protective coating?
3. If I did do an acid etching and then do the Scotch-Brite would it defeat the purpose of the etched surface? I.e. the sanding would just take off the white color and leave the brushed aluminum look again?

Sorry if the questions are common sense to those of you who know your stuff, it is surely not common sense to me.

Thanks for your help!

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welder
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Re: Scotch-Brite Aluminum Hull

#2

Post by welder » Tue May 31, 2016 9:15 pm

Aluminum starts to Oxidize within seconds of it's protective layer being removed.
If you want to keep it pretty looking get some sharkhide and follow directions.
If you want it to take its natural course then just let it turn the dark gray.
Lester,
PacificV2325, Honda BF225
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sirrox
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Re: Scotch-Brite Aluminum Hull

#3

Post by sirrox » Wed Jun 01, 2016 2:05 pm

Thanks welder, that is what I have read. I do understand it 'starts' to oxidize immediately, but how long does it take to establish a good protective oxidized surface to protect itself from staining? I would suppose the level of protection it achieves is likley a product of its environment (humidity, heat etc.)

It only had 6 days to oxidize before I traveled to Alaska and put it in the ocean for 2 days. Based on the staining of the area I sanded and no staining anywhere else leads me to believe it had not sufficiently oxidized yet to protect it self from the staining like the rest of the hull. But the rest of the hull had no issue with staining.

I guess I can try leaving it for a month and just do some fresh water trips before I dump it in the salt again. See how it reacts to that. If it doesn't hold up, I'll scotch-brite it again and sharkhide may be the answer.

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welder
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Re: Scotch-Brite Aluminum Hull

#4

Post by welder » Wed Jun 01, 2016 8:07 pm

Fresh or salt you will still get the scum line on the hull.
My hull is almost 10 year old and I get see the water line, Read, different shade of gray.
Lester,
PacificV2325, Honda BF225
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kmorin
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Re: Scotch-Brite Aluminum Hull

#5

Post by kmorin » Wed Jun 01, 2016 11:59 pm

welder, sirrox, all water, fresh or salt, with the exception of distilled; has some metal dissolved in it- that is often in the form of a 'salt' or a result of an acid and base reacting. Now we're not talking metals like the boat is made from that have been super-duper refined to one huge big piece of the same molecules- we're talking; Engine oil that has steel grindings, the are powdered, the run out the dipping seals in a McDonnie's parking lot, rain rinsed into the Texas rivers with the storm, the oil floats off leaving the powder to rust becoming mildly alkaline and that meets some run off that is mildly acidic. The combinations are so endless in our world it approaches endless to discuss.

This steel is reduced to a ferric oxide 'salt' with more junk bonded in than a research chemist could isolate in a week. THAT is a small percent of the water - in solution. Small equals ~0.00000001% or something really small- so there is very low reactivity to the water, we drink it. It won't hurt your gut even if beer is tastier.

Now, etch aluminum to a fine dull grey- I prefer the white metal look personally- and you've prepared THE most bond-able surface for the water's metal salts that you can imagine! Different areas, different minerals in the water- both salt and fresh- do the same thing to bare aluminum. Some places are blue stains, some green other a very dark brown or grey to almost black. I 've see a pic of on almost yellowish stain, like Allodyne. But all these reactions are to a film that is so thin (0.0001") that it can actually be wiped off in some cases.

So that's a quick look at a rough description of the chemistry, avoiding the list of metal salts and other reactive compounds. LIke welder says, let it go or try to work some coating into cover and plan to periodically redo the coating. Just don't go boating in the copper mine's tailing pond.

Cheers,
Kevin Morin
Kenai, AK
kmorin

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