Surface marks, corrosion?

[Katoh]

Colleagues
On my journey of rebuilding my tub, I have decided to total strip the bottom plates back to raw Aluminium and then ultra sound them to find any inner corrosion or bad areas.
with the paint removed and a quick sand I have noticed these areas which appear to be some sort of surface corrosion. The areas are were the previous paint had bubbles and peeled off. I can sand these out, and the majority seem to be only .1 to .3mm in depth from the surface.
Now my question's are what exactly am I dealing with, is it a corrosion or a more of a pigment?
Should I sand these areas out, or is there some sort of treatment for them prior to new finish coats?
The aluminium plate is 5mm, what would be a safe amount of removal and still maintain integrity?
added is a photo of a typical area, this would be say approx 150mm x 100mm and situated just above the keel.

[michael pierzga]

Looks to me like the typical corrosion found on any aluminium boat. for surface prep., sandblasting is king. contact your local marine paint rep. and inquire about what system of surface prep and coating is recommended.

Do make sure that your anode field is correct and that you have no stray electricity that may be shortening your coating systems life.

[Katoh]

Hi Michael
Two reasons I cant sand blast, one the hull is not mobile, I would have to find a mobile sandblaster, the other correct me if I am wrong but wont sandblasting contaminate the aluminium?
There was some very poor wiring on the boat, some negatives had been run directly to the hull not helping matters much. The other thing I had noticed that at some stage the hull had been re-sprayed indicated buy the varying amounts of different paint layers in areas, but I have a faint suspicion that the primer used was a red oxide metal primer, I have not come come across reddish coloured aluminium primer before.

[michael pierzga]

Many times there are practical reasons why you cant sandblast an aluminium boat or component. Both Awlgrip and International make high quality primers for metal. As you can read from the applicators guides, a grease free surface and mechanical abrasion will work. Difficult to remove the aluminum pitting with sandpaper. This is one of the reasons blasting is always best. Contact your local paint rep and see what he recommends for your situation..

http://www.awlgrip.com/Product%20Dat...1+A+eng+A4.pdf

http://www.yachtpaint.com/Literature...tb-usa-eng.pdf

[viking north]

Katoh, Have you tried a stainless steel wire brush -these come in fine -med -course cup type that screw onto the arbour of a grinder. I have used them very succesfully on alum. hulls. Also alum. impregnated two part epoxy repair sticks are available to repair and fill alum. I used same on my alum. hull and had no problems over the 9 yrs. i owned it. Be careful in your selection of sanding paper also some compounds react with alum. Alum. primer is usually classified as an etching primer and most companies match their finishing products to their primer. I generally use International paint products who also supply great printed info and intructions.--Geo.

[Katoh]

I have actually looked for the stainless brush grinder fittings but no one around here seems to stock them, thier either steel or brass, two things I dont want to brush the hull with.
I have some fabulous top coats already, and plan to use PreKote refer
http://aeroparts.com.au/shop/index.p...a5616417389241
I am just not sure what to do with those spots. I plan to paint stripper the bottom plates aswell, and then high pressure wash just remove any indented paint or paint that I may have missed. A flap disk on the grinder will make short work of those marks but I'm removing material as well, Is there some sort of converter out there like The rust converters you use on steel that are made for aluminium?

[michael pierzga]

Sure, they have surface washes, etches.. Best to only use the wash, etching products according to the paint manufacturers recommendations. You would not normally etch prime an entire hull. I never use the stuff for aluminium repairs, only degrease, grind, mechanically prepare the surface and I have had good results.

And call around..there may be a sandblaster in your backyard. Check with an auto or farm machinery repair shop.

[viking north]

I have but only on a rebuild and paint--International paint rep. recommended it --Very stinky toxic stuff to work with, however i had no paint seperation . As Michael mentioned search around -there should be someone in the mobile sand blasting buisness- had my recent build (glass) blasted to remove paint by a mobile unit. They used soda ash to remove the paint while reduce removing hull material. Not sure if soda ash is condusive to alum. I prev. used paint removers and a hi-pressure steam genny on alum.

[messabout]

A common preparation for aluminum is a dilute solution of phosphoric acid. There are numerous commercial products for that use. One is called Ospho. (your dentist may also use phos acid to etch your tooth prior to installing a filling) Consult your local automotive paint store for that stuff. (no not the detal grade) They will also have a paint primer that is described as etching primer. Properly applied it is near bulletproof but it is not pretty....yellowish blah colors. I suspect that the marine paint manufacturers use pretty much the same formulations.

Stainless brushes are used frequently by welders who do aluminum. Try a welding supply house. If that fails then try MSC industrial supply. 1-800-521-9520 or MSCmetalworking.com They have a 2300 page catalogue of industrial grade goodies. If you buy something from them, you can often have it by the next day without having to pay for UPS hurry up service.

[viking north]

Yes Messabout that was the primer, a sick yellowish green. I applied it out in the open air with a chemical cannister respirator and I still tasted the dam stuff for two days later. You could smell it downwind for two hundred feet-- but it did the job. Katoh sorry i didn't mention,I get my stainless cup brushes from a welding supply-I am fortunate living close to an industrial supply park unlike my early days in boatbuilding when I lived on the south west coast of Newfoundland when most speciality items had to be ordered in. Ahh but in those good old days, one could always find reasonably priced helping hands and a Friday afternoon shop party Thats when I scheduled the heavy lifting as it was well known the shop would be stocked up on beer. The friday shop rule was posted-- Beef( muscle) before beer.

[Poida]

Hi Katoh

We do heaps of aluminium welding and do not use stainless wire brushes.

Yes it does seem to be difficult to get them in Ozz.

But if you find them in an American Web Site postage from America is not expensive I get packages from them often. And speed, I can get a package from America faster than a letter from Queensland. 1 week from America, 12 days from Queensland.

Them Damn Yankees and their Pony Express Yeeehaw.

[Katoh]

A common preparation for aluminum is a dilute solution of phosphoric acid. There are numerous commercial products for that use. One is called Ospho

I wonder if that's the same stuff that use as rust converter on metal? it too has phosphoric acid.

Thats when I scheduled the heavy lifting as it was well known the shop would be stocked up on beer. The friday shop rule was posted-- Beef( muscle) before beer.

The good old beer economy I love it, built half a house on it once. mind you that's how I got the boat rolled onto its side now, 8 blokes and beer plus egg and bacon rolls for the boys.

We do heaps of aluminium welding and do not use stainless wire brushes.

Did you mistype that, you do or you don't? Don't you brush the Al with stainless brush prior to welding? I can't see the stainless brush on the grinder being a bad thing if anything it will definitely clean any surface contaminates up that I may have missed.
No need for sand blaster I have the paint off the bottom plates already just a quick clean up with a brush and a high pressure blast and ready to go.
I do not see the point in totally stripping the top plates as well, they show no corrosion or paint peeling whatsoever. I will scuff them up with some course paper high pressure clean and spray a couple of new top coats over to freshen them up.
Thanks for all the responses.