Paul said:

> I pour gasoline onto the part and watch the foam just dissappear.   (Unless you screw up and use fuel-proof foam). 

If the foam was really soft and “crumbly” (that’s the technical term) – and it’s a “craft” foam – then you are probably using a Urethane foam.  Easy to shape – and solvent resistant. You can give it a shot – but I’m guessing gasoline won’t touch it.  I usually use leftover scraps of my wing foam (Styrofoam).  It dissolves quite nicely with any old solvent – or I just start digging with a broad wood chisel and pry chunks of foam out.