Rubbing Alcohol works great to thin epoxy. Be sure to use alcohol that is as close to pure as you can because water (commonly used as a dilutant in rubbing alcohol) significantly slows the cure time.

tlcdennis,
Are you planning to paint a raw foam model?

For what its worth, covering the model first with either packing tape or a low-temp, iron-on covering (such as TowerKote or EconoKote) will not only provide a fuel-proof barrier over the foam, it will also make the model MUCH stronger.

HTH,
D