Every ones advise above is good. One thing you can do is take tubes of Epoxy cement and mix them with fiberglass cloth cut up into small 1/4 inch slivers. Add the slivers of fiberglass to the epoxy cement until it becomes a paste and apply that instead of body filler. It is significantly stronger than the body filler and will take a little longer to begin to bubble. It WILL NOT give you a permanent fix. Just last a little longer.
Do not use polyester resin. It is not strong enough. Use Epoxy cement or Epoxy resin only.
I hate to admit it but I did this in the 70's as a 20 something year old and got a few years out of an old Merc I had.