Originally Posted by Eagle202
If the dent was smaller the foam has a memory and a little heat from a hair dryer or heat gun will help it expand back to normal. You just have to watch that you don't apply a lot of high heat or the surface will start to "popcorn".
The size of that dent requires the steps that NorCalMatCat, Dkumagai and AJ gave you. Really warm water, filler then shape it and tape it.
I actually had not heard of the Microbaloons that AJ mentions and will get myself some. I can always use something to patch and shape my foamy.
Microbaloons work very well as a filler when mixed with epoxy. It cures like plastic foam depending on how much you add to the epoxy.
A word of caution: Microbaloons are tiny glass beads, DO NOT INHALE THEM. Wear a mask as these can be as damaging as carbon fiber dust to your lungs.