pbwe,
If you go to Robar, go to the FAQ section and there
it specifies to what metals NP3 can be applied.
It can be applied to aluminum and stainless steel.
Also go to kaneohegs.com, once there, go to
equipment test and evaluation, scroll down to
Robar coatings technologies NP3, they show a
Sig P226 with NP3 with detail explanations and
pictures.
Like I mentioned before I have a few buddy's who have
done this to some of their guns.
I also happen to order an AR 15 upper and lower with
NP3 coating coming next week and those parts are
made from 7075 T6 aircraft aluminum.