what kind of coat do you use for covering metal after airbrushing it with createx paint?

Answer 1

Use a finishing coat

Answer 2

Several factors to consider.
How much abuse will the finished item to subjected to
something kept inside the house doesn't need the
protection of something outside.
Be sure what you use has the same vehicular base of the
paint you used. A stronger solvent will cause buckling in the
base paint.
If you are unsure, do a test area on something, paint on metal
let it dry and then put on finish and see what happens.
Or use something recommended by the manufacturer of the
base paint.

Answer 3

Check with the manufacturer and examine MSDS information. Not all paints are created equally and even though they wash off, that does not mean they are appropriate to be spray tattooed onto the body, particularly the face as the case with a lot of temp tattoos.