I have heard that I should use polyester filler instead of bondo due to its resistance to cracking. However when I go to the stores all I see is regular cans of bondo. I dont know of any body supply houses around here(there may be some that I dont know about), so for normal rust spots that arent all the way through, would the normal bondo be ok. Also if I'm working with a panel(door skin) that has already been bondo'd and the paint has peeled and the bondo is showing, can I sand down that bad area and clean the old bondo good, put new bondo over old and get good results? Or will I have to strip it down all the way?

Also I believe I have a rust hole underneath my t-top weather stripping right in the middle at the front, so what should I use to fill this in considering that it will be constantly exposed to water?

My doors have a lot of rust on inside around the weld seam of the inner and outer skins. Would it be ok to strip this area completely down, and coat with POR15 even if the welds are stripped away in the process? Will the POR15 hold the door together sufficiently? Here are a few pics of what the doors look like, the last shows the area where paint peeled and bondo is showing plus the rust which wraps around to the inside:

 

I could be wrong but im pretty sure bondo is a polyester filler, and should work just fine. And as for the doors, I would probably buy a door skin or even just get a door from an autowrecker and transfer your stuff over to it rather than mess with that rust