as Duracoater said, prep work is the biggest factor in painting a gun stock or anything for that matter. you have to determine what's needed for the particular material you want to paint, whether it be metal or synthetic.
another thing to consider is the composition of the synthetic stock. some are just cheap plastic and are harder to prep than others.
and my final wipe-down is done with alcohol right before i apply the first coats of paint. learned that from painting plastic car parts over the years.
anothr tip. before even scuffing or sanding the stock or metal, wipe it down with brake cleaner, acetone or alcohol first. any waxes, oils or contaminates will be imbedded and can ruin the finish if this isn't done. also when i paint a synthetic stock, i use 3M red scuff pads, and use them wet with water and Dawn dishwahing soap. i then dry them completely and then wipe down with alcohol and then it's ready to paint.
another tip when painting. use more light coats and allow ample time between coats. more light coats with ample drying time in between are are much better than fewer heavier coats.