I modified a tupperware stock heavily using bondo. Did a lot of mod with it. Have not had an issue with it not sticking or coming loose over time. Have shot 243 and 308 in this stock. It has been sitting doing nothing for some time because of mistakes I made in not keeping the bottom of the forearm level with the rest of the stock. There have been reports that the bondo will crack over time, I don't know this personally. It may be worthwhile to cover with a layer if fiberglass and resin.
Good luck with your project
Bookmarks