How can I paint my rusted steel barbell weight plates after galvanizing them?

I have to just shake my head at this.

Spray galvanizing compound is not intended to be painted over. It's not a primer. It's basically just powdered zinc, with a small amount of latex binder so it'll stick to the surface. It's meant to be applied to bare metal, and then left that way.

In any case, painting an object makes galvanizing totally unnecessary and a waste of time.

What you need to do is clean the galvanizing compound off completely with lacquer thinner.

Then, get a can of REAL primer. As I mentioned before, spray galvanizing is not primer, you shouldn't be surprised that your paint didn't stick to it.

I'd also recommend lightly sanding, since there'll probably still be a thin film of compound fouling up the surface.

IF the rust is superficial, then use that wire brush and some sandpaper to get it down to the metal. Then use a good metal primer on the entire trailer. Go right over the paint that is already there. Give it two days to really dry well. Then use an EPOXY-based paint. Or, if you can find the facility that does it, go to a powder application epoxy facility, and get it powder-coated with epoxy. But in either case, use an epoxy based paint. Allow a week for complete setup of the paint, and you will have a trailer that will never rust again!! Enjoy!!

You can't beat powder coat for a tough finish. But that would mean sandblasting the steel and then a professional paint shop coating them.

Presuming you wanted to paint them yourself, you want a water based epoxy paint.

probably best to just start over.

go to your local hardware store and get some paint remover, since your paint is still kind of fresh it should come off easy, the paint remover is usually a thick gel type substance that you paint on and let it set, the paint will bubble up and you can then wipe it off and repaint with the right paint.