Standard tests for absence of pores (copper sulphate or cadmium sulphate method, determination of breakdown voltage) will also provide information about gas impermeability. There are special instruments for accurately measuring this on the actual paint film. It should be fairly easy to find a filler which will not affect the whiteness of a white-pigmented paint film.
Suitable products include kaolin, blanch fixes as well as whiting, although there is usually the risk of a reduction in gloss. In that case the filler concentration must be reduced. As far as adhesion is concerned, epoxy resins must be blended with phenolic or amino resins. Two—coat application might also help — i.e. a flexible primer, followed by the actual paint.