When I had my bumper filled the body shop roughed up the cbra letters with a grinder and put some mesh tape in there too to help the filler material bond better.
The filler material is a binary polyurethane compund (it is slightly stiffer than the stock bumper material) . There were two tubes (one was the filler material and the other a catalyst material) whose contents was dispensed (from what could be described as a caulk type gun) out of one nozzle onto the prepared letters. The mixture was smoothed onto the bumper with a piece of plastice the size of a credit card. The filler quicky dryed. This process was repeated untill all letters were filled.
After the compound was completely dry, it was taken down with a grinder, then sanded smooth. It was next cleaned, and prepped for paint with primer and shot with the base color and clear coat.
Don't use bondo on this project. I don't know what tradename the binary polyurethane material went by. If your bodyshop doesn't use it and wants instead to bondo....move on.
My bumper work cost $500.
Hop