Thread: Which etch primer for Aluminium?

Most aluminium has a coating on it to stop it corroding, so you will need to sand it off before painting, otherwise it will just sit on top of the coating. Even for polishing I have to get under this coating to get anywhere with polishing.

When I go to my sand blasters he often has smaller aluminium boats their getting blasted and painted, so the etching would hold it on.
If the part is only small you could sand blast it with a cheap $30 kit, but of coarse you would then need a compressor.

The only other thing I can think of is contact a boating place and ask them what type of etch or primer to use.

Dave

Steel wool on aluminium, hmmm, definitely not recommended. Scotchbrite is the way to go.

Not sure what's out there today, but had good luck with alodine followed by zinc chromate in the past prior to painting. I understand the chromate is now considered a carcinogen and might be harder to come by.

ZINC CHROMATE is the prefered primer for aluminium. The aircraft industry uses it extensively . Try your local airport maintenance hanger. It's a light green colour..you apply a single ultra thin coat, the finish is opaque .

On my old Land Rover ( aluminium body ), I used the etch primer stuff in a pressure pack from Super crap auto ...you apply only one thin coat ..its grey coloured . It seems to have worked OK . Good Luck .MIKE

Thanks for the replies everyone.

One last question (pardon my ignorance).... why is steel wool bad for cleaning aluminium? And with other methods, like scotchbrite, how do you know when it's clean enough? (I'd just assumed that when it's nice and shiny bright, then it's ok to apply the primer.)

there is a good article here on aluminium prep
steelwool leaves metal particle in the aluminium which startes corrosion
be careful sandblasting also as you will distort thin aluminium very quickly

cheers
Harty

Originally Posted by harty69

there is a good article here on aluminium prep
steelwool leaves metal particle in the aluminium which startes corrosion
be careful sandblasting also as you will distort thin aluminium very quickly

Thanks. That's an interesting thread.

I only have a small quantity to do, so I won't be sandblasting. Maybe I'll go the vinegar+ZC mentioned later in thread as a 2nd-best approach. If I can find a ZC rattle can, that is.

Now that I've read a little about this, I'm really surprised that manufacturers of general-purpose etch primer are allowed to say that their product is suitable for Aluminium. Seems like total BS.

Cheers.

Originally Posted by strangerep

And with other methods, like scotchbrite, how do you know when it's clean enough?

You can get al shiny with scotchbrite too. Might mention, there's grades where you don't want to attack the surface, as some have a soft protective pure aluminium coat. I assume you're using typical hardware store 5 series though.

Don't want to tell you how to suck eggs, but you've given it a wipe over with Prepsol or similar to remove oils and waxes?

Originally Posted by Hunch

Don't want to tell you how to suck eggs,

Nah, that's fine -- my knowledge of metal work is rather minimal.

but you've given it a wipe over with Prepsol or similar to remove oils and waxes?

All I did was clean it with steel wool and sugar soap. Sounds like I should try prep'ing it a bit more seriously before bitching about general-purpose etch primers.

Cheers.

Prepsol is cheap at around $20 for 4lt or you can buy it in smaller quantities. Even the oil off your hand can affect the paint sticking.
Once you have a tin you will find other uses for it as well.

Dave

Zinc Chromate primer is certainly the best thing I know of for aluminium. Get some good PPG or Valspar stuff and it will do the job admirably. We use it on various aircraft parts and repairs. Excellent for corrosion prevention. However, treat it like liquid cancer, because it pretty much is. Respirator on before you even look at the tin, and be careful of any drift from the overspray, it can waft around for a while.

As for prep, the protective coating on alloys is usually a layer of pure aluminium that forms an oxide layer and protects the inner, vulnerable alloy. All you need to do is scuff the surface with scotchbrite until there's a uniformly brushed look to it. Ultimately, one would then dip the part in chromic acid to etch the surface, then a rinse in fresh water followed by a treatment of Alodine or similar product. Those last two are likely difficult to obtain outside of industry and for all intensive purposes could be omitted for around the home jobs. A good wipe with a lint free rag and thinners to rid the surface of any oils and contaminants. Usually dust a coat on and then spray until full colour on the part. It is pretty versatile stuff, so the amount you put on isn't ultra critical. A very light scuff to remove any bits from the surface after its dry and then onto topcoat. Be aware though that some topcoats don't like sticking to two pack primers, so perhaps do a check first.

This is all observations from my own experiences using the stuff, so if I've made any blunders, let me know.

Cheerio, Tom.