For this purpose I use micro balloons a brownish powder.
First I cut the seams open with a sharp utility knife and for tricky places I use the pneumatic
straight router. It is like a Dremel tool, I chose this version since I burnt out a Dremel (a copy though) doing carbon fiber. The carbon got stuck in the bushings I believe.
Then using a candy bag I apply the bog two times with no sanding in between ( apply second layer in less than 12 hours).
Sanded and ready for laminate. Note the 5 cm edge of bog at the perimeter of the hull. This is for the vacuum tape. I do not trust that the foam will make a airtight glue with the tape, although other builder do without. But not much extra work to be sure.