Aaron_F

I'm painting an old steel frame, but not sure if I should sand the primer before painting. The primer is pretty rough to the touch now that it's on the frame, but if I sand it, is there anything for the paint to hold onto?

Doug Fattic 

Yes, sand the primer throughly. It is one of the steps that will give you a better final result. If your color coats are a urethane like Imron, I would use a 320 grit dry paper (wet is okay too). I forget the model name of the sandpaper I use from 3M but it is a gold color. I used to use a white paper they made called Tri-m-it. I like to cut up a full sized sheet into small rectangles. Usually that means I keep cutting the paper in half until they are 1/16th the original size. I get a tube or two out of one of those small pieces before I throw it away for a fresh one. If I am using a base coat/clear coat system then I sand the primer with a wet/dry 600 grit black paper. In this case I sand it wet. This finer grit is necessary because base coat colors (like a House of Kolor Shimrin) are thin and will show the sanding lines of courser paper.

Aaron_F

Great advice, thanks!

squirtdad

Based on painting 2 whole frames (one rattle can, the other automotive paint (color and catalysed urethane clear using a preval sprayer) if there are any bumps or edges in the primer or if it goes through to bare metal it will show up in the final finish. The guide I followed (mostly) was to use metal etch primer on bare metal, then follow with a high fill primer. The idea was then to do a light second spary of contrasting color primer over the high fill and sand until the contrasting color was all gone to ensure even sanding. Didn't follow that 100% and can see where i didn't

have fun with the paint job

pyeyo

Each coat "bites" into the one before it which is why paint manufacturers list recoat times, if too much time passes a light scuff with fine grit paper is usually all that is required to get it ready, always use a tack rag before painting each coat. As mentioned above glamour paints like shimrin do not have a high hiding ability and finish differently depending on the color of primer/sealer beneath them, some candy colors need to be sprayed over gold or silver for instance, [some colors are better than others at hiding, lighter colors typically poorer than darker]
The above advice is very good if you do not go down to bare metal. Read up a little to make sure your paint[s] are compatable with each other, i.e. you typically don't spray lacquer over enamel or something really "hot" like a catalyzed product.
If you intend to mask go to an automotive paint retailer for their recommendations, many time I use an interclear to not lift the under color before intricate masking.
Some of the custom car paint treatments were discovered accidently be mixing the wrong type of reducer with the paint causing crazing, crackling, and other things. Cobwebbing came about by spraying unthinned paint.
Make sure you have fresh air or possibly a carbon filter mask if you are in a confined space, some toothpicks to pick out bugs, and gloves. Blow off the bike before painting with an air nozzle to insure their isn't any trapped water or gunk, it really is upsetting to make a pass and have some junk come out of a brake boss. Thread a bolt through the derailleur hanger so you use it as a handle to spin it in different positions, sacrifice some bolts for bosses and hangers to keep threads clean, I slip those magazine subscription things that fall out of magazines while your reading them in the bathtub into the bottom bracket and head tube to keep them getting build up inside.
Heh, good luck and post your result.