spider007
Be careful... there are some tricks/traps here: Powder coatings may damage, or eventually damage, aerospace parts.
1. Cure Temperatures.
MIL-C-24714 allows cure temsp of ~400F for ~30 Minutes.
AMS3143 allows cure temps in (2) catagories (no time limit specified]:
Class 1 NO restrictions.
Class 2: 300F [no time specified.
These temperature ranges can affect temper of some 2XXX and most 7XXX aluminums.
2. Coating thickness: Powder coats tend to routinely exceed 0.003" dry-film thickness; wheras production-quality solvent or water-borner primer coats of 0.0006" dry-film thickness are common. In the case of epoxy coatings: LESS is superior to thicker, since flexure of thick coatings causes brittle cracks to open and poor adhesion of any decoratve top coats. Obviously "stiff parts" [forgings, thick extrusions or plate] are far less affected that sheet-metal [in flexure].
3. There are several aother critical issues regarding powder-coating of aerospace parts:
3.1 These temps can cause serious outgassing in porous castings [aluminum or magnesium]; so-much-so that finish quality is poor [blisters, pin-holes, etc. In some cases powder additives can help control these blemishes... but NOT 100%.
3.2 "Old" unsealed magnesium and aluminum castings [+10yrs in service] are subject to low-level moisture intrusion: this can either desrtroy the casting if heated to "moisture steam" point [crack/explode]; or can cause totally unreliable coating quality due to out-gassing.
3.3 Thick/hard/baked coatings are typically VERY difficult to REMOVE [strip-off] using chemical or blast-type stripping methods.
3.3.1 Chemical strippers have to "work" longer and harder to break high-temp cure bonds... especially when very thick. In some cases special strippers would have to be formulated for removal of specifc epoxy powder-coating formulations. In discussions with a paint stripper vendor, I expressed concern regarding removal of finishes on adhesive bonded thin-skinned aluminum-honeycomb parts. I did NOT want the exterior adhesive bond-primer damaged... while stripping thh Polyurethane top-coat and corrosion protective epoxy primer. Vendor assured me that the baked epoxy adhesive bonding primer had MUCH tougher chemistry than the solvent or waterborne corrosion protective primer... and sure-enough [after several tests] we discovered he was correct: the chemical stripping action seemed to "stop" as it hit the baked material.
3.3.2 Media or water blasting of hard resinous surfaces over soft aluminum or magnesium substrates is difficult at best; and very damaging to substrates at worst [media entrapment/contamination in surface, surface hardening and scoring].
Regards, Wil Taylor
