Check this thread out: http://www.talkcomposites.com/56/Foam-Pattern-to-a-Gloss-Finish

"Sure. The ratios arn't too critical with either the bubbles or the silica and will really depend on the consitancy you want the 'pattern coat' to be. The more bubbles you add, the more sandable the coat will be but the less tough it will be. The more silica you add the thicker it will be (the bubbles will also make it thicker too). It's a case of adding until it seems about right. I would guess by weight only maybe 10% bubbles and a few percent silica. The thing to watch out for with the silica is that it takes a while to 'take effect' on the resin because the crystals take time to absorb the resin (several hours to reach full properties) and also a decent amount of agitation so my suggestion would be to add a few percent by weight to the resin (with no hardener added) first and let them soak in the resin for a few hours before giving it a really good stir. You'll be able to guage how thixotropic it then is and then add your bubbles and hardener once you're happy."

Dave.

i'm guessing you want to make the epoxy resin more thixotropic?

Can anyone suggest a good mix ratio for a foam pattern sealer?

I understand I need

Epoxy Coating Resin
Glass bubbles
Silica
Could do with some graphite to give a black colour

Many Thanks,
James