Oh my FUCKING gosh! I just typed a REALLY detailed reply and everything got erased! Grrrrrrrrrrrrrrrrrrrrr! I'm not doing that again, but yeah...
Daaaaaaamn, that is some SERIOUS rust action, though! lol I just saw your old thread. Rust is like cancer -- it spreads.
Media blasting can work wonders for removing paint and rust. In your case, I think that spot should have been cut out and a new panel patched in place of it. That's not to say that it's the only method. Just the best in your case, in my opinion. When patching panels, you have to be careful because the heat from the welding process causes oxidation and can leave the new area even more prone to rusting than before. If you decide to do it again yourself, make sure you treat the new welds and areas around it properly to rust-proof it, otherwise you'll end up causing more damage than repairing it.
Plastic filler -
How much of it did you use?
Plastic fillers (like Bondo) should only be applied about 1/4" thick (twice the amount you'll want actually staying on your car). That'll give you room to sand and shape it to about 1/8" in thickness. Anything thicker than 1/8" will be more prone to cracking and falling off due to heat, flexing, etc. later on down the road. That's what I had originally thought was happening to your car now. lol I'm not sure that's the issue anymore, though. Oh, and just for reference, plastic fillers should not be used on areas that are subject to excessive flexing (hood, hatch, doors, etc.) for the same reasons.
Removing the rust -
Are you sure you removed all of the rust prior to making the repair? Did you sand and treat the metal?
If you opt to repair it yourself and avoid patching panels, you have to take all the necessary steps to be sure that you've removed all the rust. I had small rust spots on my car. To remove the rust, I sanded all the surface rust off down to bare metal (areas around it as well) and treated the entire area with a rust treatment. Then I took a stiff wire wheel to the area to make sure I got to any rust hiding in the pores. Sometimes the area looks rust-free, when in fact, the rust is still there. It just might not be as visible to the naked eye anymore. Only after you're sure that you've treated the area as best you can, should you start attempting repair it.
