This is probably what he's using.
https://www.axalta.com/content/dam/NA/HQ/Public/Nason/Documents/TDS/NSN-TDS-422-5X-Eng.pdf
This is what anotheridiot is referring to:
https://www.axalta.com/content/dam/NA/HQ/Public/Nason/Documents/TDS/NSN-TDS-421-10-Eng.pdf
Couple of ways to do it. The correct way is epoxy bare metal then proceed. But it sounds like this guy is doing his own prep work, so who knows what's going on with it. If this is a low end job, very small sand throughs at the edges of panels you can approach it like this. This is typical of collision repair work. Get everything clean, masked, and wiped down. SEM etch primer in a spray can, dust a couple of coats on the bare metal. Seal it and proceed.
If the bare metal spots are in the middle of a panel, larger than a quarter, or there are numerous spots, then you're going to have to do more. Spots not on the edge are going to show through. Type of job that it is (guy doing his own prep work) you can either spray epoxy, reduced epoxy. or 2k urethane. Being that the truck probably doesn't have epoxy on it and that you are probably not making much on it, you could 2k those areas and wet sand them. It's done all the time in collision repair. Not saying it is correct but that's the reality. Or you can reduce some epoxy and shoot that. Light wet sand or light hand sand dry with 400 dry paper, then proceed. Or full epoxy and proceed.
If I was doing it, I'd try to etch prime first like I described. Then honestly if it's the type of job I think it is, if it needed more I would 2K it, wet sand it and proceed. Obviously not correct but seeing you didn't have control of the prep on the vehicle you don't have any idea how well the prep was done to start with.