I just used the Bondo scrapers and a new to me product West Systems G-Flex Epoxy to fair out scratched up Kayaks. It is an excellent product and very tough as it retains some flexibility. The Bondo spreaders I think are silicone rubber and cleaned easily with Isopropyl alcohol. white vinegar or acetone and then soap and water.

I think the G-Flex with or with out micro balloons is the way to go to fill and fair a synthetic stock. It would also work for bedding the barrel channel while using something harder for the action.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli