from "finishing a composite airplane"
"I strongly recommend spraying a light coat of black primer, before you start filling, Even thou this step is not absolutely necessary it is very helpful in guiding where to direct more sanding and when to change to finer sandpaper grade. The filler becomes translucent as it gets thinner and the black color starts showing through as dark spots indicating you are getting close to the skin surface."" I mix blue pthalate dye with my filler for the same reason. I question the idea of having primer in the middle of layers of epoxy. Probably very minor. Bob P.
