I use clear Kiwi shoe polish for all my bedding jobs and never had a glue in. It is cheap, might be cheaper than Pam, and lasts forever. Also, use blue painters tape to mask off areas you don't want the bedding compound squeezing into. To get bedding compound to stick to the plastic stock it needs to be absolutely clean. I suspect there still is release agent on the stock from when it was molded. Make sure to clean it really well using acetone. Next I woul use very course sandpaper to roughen the area where the bedding compound is being applied. Normally this is enough for the bedding compound to stick to synthetic stocks.

There are two problems with JB Weld as a bedding agent. First is that is tends to run into areas you don't want it in. Second, it is generally to brittle. I have used JB Weld in the past because it was cheap and easily found but I quit using it a LONG time ago. If you are having problems of it running into areas you don't want it get some modelers clay or blue painters tape from WM to keep it out.

Devcon Steel Putty, as GaCop said, great stuff and the only stuff I use anymore. It is thick enough to stay where you put it. It is tough without being brittle like JB Weld. Finding Devcon products can be hard unless you order it but there is an alternative that can be found in almost every boat shop. Marinetex is another product people use to bed rifle with regulary. I have not personally used it but it has to work if that many people swear by it.

Dolomite