With Monokote, it's easy. Just use Top-Flite "Trim Solvent" instead of heat. Acetone will work too, but it's more agressive than Trim Solvent, and gives you less time to work.

Neither works on Ultracote

For Ultracote, I don't usually have a problem with small or narrow pieces, but for larger pieces I will either remove some of the covering underneath, or poke bazillions of holes in the underlayer to let the air out.