I have good luck using DAP vinyl spackle (not the light stuff). I simply smear in the dap on all of the pinholes and sand off. DAP shrinks when it dires, so it doesn't work too well for large craters. Then I use a cheap primer such as 30s and spray a light coat to see if I have any imprefections left. I repeat the process until all pin holes are filled in. If you have a glow engine, you need to sand off all the 30s primer as it is not fuel proof. After that I use the PPG primer.