I didn't have to bond any panels so can't help you there.
I did however use the P40 as a filler and also to build up on the front of the doors and around the bonnet.
It is very easy (if messy) to use. I used it instead of filler whenever I could to try and stop any shrinkage. So far (early days) I have no shrinkage.
I is harder to sand back down but I used a coarse paper on an electric sander so it wasn't too bad. it then needed only a tiny bit of filler to finish, I mainly used Dolphin Glaze as it needed so little.
Just don't put too much hardner in or try to do too large an area at once and the heat build up won't be a problem.