If it´s just a few spots, spot puddy (primer in a tube) and paint. Most places sell primer/sealer, may have to sand it some, reprime and paint again.
Tape some plastic sheeting on the good spots, so the overspray doesn´t mess it up. Tape lightly (don´t press) and get the tape off soon, might want to try a test patch, it would be a bumber if the tape lifted your paint.
Some paints, give trouble, when they become too thick. Spraying another coat, when they are dried, but not really cured, will lift, the paint underneath or cause it to orange peal/crinkle. Better to spray, thin coats at 20 min to half hour intervals, to build up thickness. Spraying the next day, can cause problems. Waiting a week or so is often preferable.
Changing brands and/or paint types can also cause problems. When you don´t get it on the first try.
I´ve learned a good primer, will hold a cheap paint. But not the other way around. I´ve had good luck, with Rustolium rusty metal primer (spray or quart can), seems to hold most everything, and most solvents won´t touch it. But it does take a while to dry (sand lightly and paint). Have also had pretty good luck with Krylon primers. Most quality automotive primers (two part, paint and hardener) are good but expensive. Spray can automotive primers, of various flavors, have given me trouble, especialy when the primer is one manufacture and the paint another.
And remember when the temp. gets below about 70 F. things seldom work right.
If all else fails, sand it off and do it again, most every paint and body guy, will tell you, been there done that.
Fiber glass, doesn´t require special primers (if it´s old/cured)and(if it´s roughed up good and cleaned with aceton or strong solvent)(be careful of aceton and paint), but if the Gel coat, is worn down or pealed off, the fibers underneath need to be sealed. They make special primers for galvinized steel and aluminum, that seem to hold a little better than normal primers (but I always seem to come back to Rustolium).
Another good trick, is to put on the primer with a roller (I often put it on thick and sand it smooth after a few days). Or put it on with a roller, and before it dries (in like 18" square patches, roll, spray, roll, spray, you get the idea) spray it with a rattle can, containing the same primer, usually ends up reasonably smooth and removes brush strokes.
I´m out of tipps, will give it some more thought.
Not related to your project, my puppy just walked up and sat in front of a puddle of urine, with a rolled up piece, of newspaper in her mouth and is looking at me, with big brown eyes. What the heck do you do now? I guess if it isn´t working, change tactics and try again.
