Auto body repair & detailing: Body Filler Cracks, lacquer paint, new cracks

Question
I have a 57 tbird that I have done bondo/body work on. It has been a few years and the bondo has held up fine while I finish other work. Recently I started my "final" sanding, block sanding, primer, block sand etc. I have recently started getting fime airline cracks in different areas on the car. By the headlight, tailights etc. I have thought that this was caused because there is the factory lead underneath but it appears to be random. I have tried different primers. I have doug out these spots and tried new bondo and/or new spot potty to fill. Some times I get new cracks in an old spot,, sometimes in a new spot. They just seem to pop up. Short of grinding out my body repairs and starting over is there any way to SEAL or stop them. I started using lacquer primer so have continued. Can I seal it with a poly primer?  

Answer
Terry- you're doing something wrong, but I don't know exactly what. I suspect you are applying filler too thick- it shouldn't be over 1/4 inch thick, ideally 1/8. Are you using a quality filler? If you didn't pay 30 bucks a gallon it's crap. I personally love USC Rage filler. Are you prepping the panels right? The panel must be ground to steel with 36 grit on a grinder- not a D.A. You can't allow the body filler to touch the old lacquer paint and primer, either- it will cause lifting. I also suspect you might be getting problems from the old paint finish. 2 part primers really don't do well over old lacquer finishes, and the panel has to be primed completely- no spot prime over lacquer with urethane primer. Be sure to use plenty of hardener in the filler- it should start hardening within 5 minutes of being mixed. If it takes any longer, you are not putting in enough hardener. Blow and wipe any and all dust off of the spot before applying filler, also. Bill