Aluminum Etch?

[Dave B]

I want to epoxy coat the hold-down tabs on my aluminum fuel tanks because they will be sandwiched between pieces of wood and I want to protect them against water intrusion/corrosion. Just the tabs will be sandwiched and the tanks themselves won't have anyplace that water could collect. (The reason for doing this is a long story, but it has to happen.)

I understand that aluminum is supposed to be acid etched before the epoxy is applied. I know that I could buy a "kit" that contains the necessary ingredients, but I'm wondering if I might have the right stuff on hand already. Does anyone know what's needed?

[MiddleAgesMan]

Phosphoric acid diluted with water. The Awlgrip folks sell something especially for this--Alumiprep 33--but the smallest size is a quart and it will be more than $30. damhikt

Using a Scotchbrite pad, scrub the aluminum with the acid mix then rinse it off. As soon as the surface is dry apply your primer.

[TerryLL]

Ospho is the product you want. Dilute phosphoric acid.

http://www.skybryte.com/

[AstoriaDave]

There is more to the Awlgrip Alumiprep 33 than just phosphoric acid. Check the MSDS here: http://www.yachtpaint.com/awlgrip/awlgrip_msds/73001.pdf

In short, Ospho may do the job, but for a critical location such as this one, I'd invest in the Alumiprep. The good news: it has no chromates in it; the bad news: it has fluoride in it, which in combination with the phosphoric acid makes hydrogen fluoride, nasty enough to etch glass.

I'm wondering whiy a layer of 5200 on each tab would not do the job. It seems to stick pretty well to aluminum, although I'm not sure about an application like this one. Further, epoxy will embrittle with time, and has a low flexure compared to the aluminum, hence the suggestion to consider a more flexible protective method.

[BrianW]

I know their website says it can be used on aluminum. But I had one huge (60x20ft barge huge) failure of non-skid to adhere to an ospho prepped surface. That barge is aluminum and after grinding down to bare metal, then coating with ospho, the non-skid peeled off in chunks days later. In fact, it failed more than once, it also failed on several aluminum barges that were extended, and needed to be re-non-skidded. (is that a word?)

We took that same nonskid and redid a couple other boats without ospho, and the job came out perfect.

Maybe it was a reaction between the paint and the ospho?? It's possible. But in my mind, if you want to acid etch aluminum, use an product specifically for that purpose.

[TerryLL]

I've used Ospho often for small paint jobs on aluminum with never a problem. But I was working in a heated shop under ideal conditions. Sounds like the Awlgrip product might be a better option for a difficult application.

[MiddleAgesMan]

Perhaps the rinsing step cannot be skipped. Alumiprep's instructions insist on it. IIRC the Ospho instructions are for applications to steel. The acid is not to be allowed to dry before rinsing, BTW.

[AstoriaDave]

Last three guys have some valuable experience with ospho and aluminum. I think the fluoride inthe commercial prep material may be the key. Fluoride ties up aluminum -- maybe keeps an active, fresh surface for a while. I agree rinsing is critical for aluminum, per the Awlgrip instructions.

Other, industrial prep materials have chromates in them -- not OK for the casual worker.

[Dave B]

After reading all the replies I did some more research and discovered that a little bit of everything seems to be used to etch the stuff. Several different acids and other additives and in a varying degree of concentrations. In all probability, many of them would work but I'd better check for something specific to the job at hand. Thanks all for the help!

[Dave B]

I'm wondering whiy a layer of 5200 on each tab would not do the job. It seems to stick pretty well to aluminum, although I'm not sure about an application like this one. Further, epoxy will embrittle with time, and has a low flexure compared to the aluminum, hence the suggestion to consider a more flexible protective method.

Dave - I had already considered just coating it with either 5200 or 4200 but my tolerances are pretty tight and I've never had any luck getting that goop to spread evenly. I need a tight, slick surface.