Try satin polyurethane to seal the pores first,then spray it with a gray primer,just enough to coat it,so that you can see it,for coverage,then paint it lightly in between coats to get even coverage. Or,you can use createx,or ceramcoat directly onto the surface,but you'll want to test coverage on scrap first,to see how it reacts with the material you're using so that there is no adverse reaction to it! There are a few websites that are geared towards this kind of paint coating,maybe they can help you out with this better than I.