if you can't see rust, you don't need primer?

Question

I was watching a youtube video on repairing paint chips and the guy said 'if you cant see rust you don't need primer.

However I notice that plastic(which doesn't develop rust) has primer layer on it. Therefore if you were painting damaged plastic body panels you would need primer and even if paint chips, the primer on the plastic may have become too thin from damage.

So seems to me his advise is incorrect? Please explain.

Since the video was about smapp paint chips, maybe he was saying in the case of those its not needed?

Answer 1

Primer helps the paint stick to the bare metal, clean or not. Primer also gives it a good base for smoothness and matching. There's really never a reason not to use primer.

Answer 2

Plastic needs to have a special primer so that the paint sticks to it.

But, for metal, even if you think it is clean it should be given a primer coat then base then top and may be a clear coat to finish, but this does depend on the type of paint.

Answer 3

I assume that in the video what he means when he says that is if paintwork is damaged but the damage isn't deep enough to have gotten through to primer, you don't need anything more than base + clear. Primer is porous so if it's exposed, rust is a typical tell-tale.

What I do not think he means is that an unpainted part of the car can be painted without primer. Typically painting plastic bumpers you'd use etch-primer to get a good key for the paint to adhere to the plastic.