Painting aluminum

No "correction" to anyone. However, in your case, provided the aluminum is not heavily oxidized, I'd prefer to use a specific chemical pretreatment prior to an epoxy primer like the MP170. I'm thinking of PPG DX-533 Cleaner and DX-503 Conditioner applied by scrubbing with a Scotch-Brite hand pad. DuPont has similar dedicated products for aluminum (225S/226S). I've used the DX-533/DX-501 combination before when directly clearcoating an engine aluminum rocker cover. Worked very well with no adhesion or other problems for several years inside a hot engine compartment. It was still in great condition when I got rid of the vehicle.

I am not a fan of combination products like etch primers. It's not that they don't work. They do. And they have a loyal following, especially in bodyshops where faster turnaround means higher profits. It's just that sometimes they have restrictions on use that I prefer to avoid. Plus, as CNKS and Frank say, most of 'em recommend another product (primer or surfacer) be applied over them prior to topcoating anyway. In any event, if a chemical pretreatment like the DX-533/503 is used, it is specifically recommended to not use any kind of additional etch primer. Epoxy, yes. Etch, no. Also I was surprised to find out not long ago (I think on this board), there is at least one lesser known epoxy primer (made by Valspar I think) out there that has some kind of "etch" (acid) function built into its' chemistry. Such a primer, even though it is called an epoxy, should not be used over an etch primer or over the above noted chemical pretreatment. As always, you should get and follow the manufacturer's specific tech sheets for each of the products you plan to use. It will minimize the possibility of problems.

Rod

I don't think I implied that most epoxies have acid. At least that was not my intention or understanding. The PPG epoxies I use (OMNI MP170 or DPLF) specifically state they may be used over PPG's etch or wash primers or, in the case of DPLF, over the DX metal treatments (phosphoric acid based). The MP170 is silent on the DX treatments since there are no similar products in the OMNI line. I have never used the MP170 over a DX treatment for aluminum, but I have used it over DuPont's 5717S/5718S phosphoric acid based pretreatment successfully on sheet steel.

I have never used DuPont's DTM epoxy primer. The time window (24 hrs) is too short for my liking. The tech sheet for it seems to be silent on the Vari-Prime etch primer but does indicate that 5717S pretreatment (acid) is an acceptable substrate. The same goes for DuPont's old Corlar epoxy, now in the fleet line. I therefore presume that neither of the DuPont epoxies contain acid but that may not be correct.

I was under the impression that most epoxy primers did not have an acid "etch" feature. That is why I was surprised to learn the Valspar product (I forget the tradename) indicated that it did and not to use it over an etch. That would presumably include either an etch primer or any acidic pretreatments.

If you understand that most epoxies contain an acid etch feature, I guess the PPG ones are more unique than I thought. I may have to revise and narrow my thinking on my favorite type of primer.

• PPG

Gene