block sanding blues

Sounds like you've used a lacquer primer surfacer, or some other thin mixture.......They have very little "solids" and tend to sand off very quickly.......If you want the problem solved for good, after you've finished with your sanding, spray the car with a 2 stage (2K) primer......It will NOT shrink, and it's very thick compared to regular primer surfacers.

Kinda' like concrete compared to cardboard in durability.