Thread: To Etch or Not To Etch...........

that is the question.


I was under the impression that you only need use etching primer on Aluminium or at least anything non ferrous. I was in an auto paint shop the other day wanting a small tin of primer for my Air Filter and I was told " you must use an etching primer if you are down to bare metal otherwise the paint will just flake off". I just used a bit of sandpaper to give it a key and an Acrylic primer. Looks fine to me. Have I done wrong?!
cheers,
D

It's not really the case that etch primer is essential on anything, what is essential is to get the primer to stick. On aluminium you can get primer to stick the same way that the factory originally did, with a chromate conversion coating like Alodine which is still available and used on aircraft. There are other chemical washes/coatings used in the aircraft industry to provide a key for paint. You can also use a suitable epoxy primer, which will work very well if you thoroughly clean and key the panel with Scotchbrite and a 50:50 meths:water mixture and then paint as soon as it is dry to prevent it developing a new oxide layer. However an etching primer (ideally in conjunction with the scotchbrite+meths cleaning) is probably the easiest approach on aluminium. If you have removed the old paint with paint stripper, and the surface still has a yellow/gold tinge, that is a chromate coating and you can normally use a traditional primer like the factory did, just use several clean cloths with lots of wax remover before you paint but don't use Scotchbrite or you'll remove the chromate.

On mild steel you absolutely do not need an etching primer, but using one will not cause problems. Galvanised steel is more difficult, etching primer will work but leaving the part to weather and lose the "bright new" zinc finish will often allow a traditional or particularly an epoxy primer to work just as well.

The main problem with steel, and in fact most substrates, is that for paint to stick (and this includes etching primers) the surface MUST be 100% free of oil, wax, silicone etc. and anything that will stop the new paint sticking or that will fall off and take the new paint with it (like old paint and rust). Normal paint prep is to wash with wax remover before and after any sanding, then wax remover and tack cloth just before painting. Wearing nitrile gloves stops your fingers from depositing oils on the part, as well as keeping the paint off your hands - if you look at a sandblasted steel item a day or two after the blasting, you can often see the fingerprints of the people who have touched the metal, their skin oils and acids develop like a brown/black photo image on the steel. By the way, don't ever try using a tack cloth on sandblasted steel, the steel will end up covered in tiny fibres that miraculously appear when the paint hits them.

If your steel air filter was thoroughly cleaned and key'ed, acrylic primer should be fine.

Also remember that most primers have a recoating window, after which they must be sanded and reprimed because they will have cured and will no longer provide a good surface for the next coat to adhere to. This window can vary from hours to months, depending on the paint.