Blue foam can be sanded and if you're careful it will yield a nicely shaped part. Gaps were filled with model filler but lightweight spackle would work. For this part I used 2 layers of .75 oz cloth put on at one time ( I didn't let the first layer cure, sand, them add the second ). After it cured I sanded with 80 grit paper to get a uniform dull surface. The high-build primer/surfacer was brushed on without thinning and sanded out. Another coat of primer slightly thinned was brushed on and wet sanded. I finished up with a white sandable auto primer in a spray can to provide a white base color which was wet sanded. If the lacquer-based primer seeps thru the fiberglass layer it will attack the foam. In this case I used two layers of cloth; another way would have been to use one layer and put another coat of epoxy after the first one cured and was lightly sanded. I didn't have time and needed to have it done with one epoxy application. Brushing the first coat of primer without thinning helps prevent the solvent from getting to the foam; the thinner would certainly get through any pinholes (ask me how I know!).
If this was a plug I would have used heavier cloth such as 2 oz. to make a sturdier part.
I also made a sample aileron to show that the process will work with control surfaces as well. It was made by glueing templates to a foam block and sanding out. Same process on the glass cloth. Quick, strong and light.
Hope this is helpful.
Paul