filler question

question is im repainting my 59 impala wings, going metal flake paint instead of stock white. took the in line sander and noticed the wings were high on the ends and low in the center. wings on a 59 are perfectly flat like a board or table










question is, is it possible to sand down the center down past the paint to the primer or existing 2 year old filler and fill with new filler, will it bond good? I just don't want to sand off existing filler and create more work when I can just use that one.


second question, car was painted 2 years ago and sat in the hot sun and rain since. but a few months after it was painted it got a crack, small one out of the whole 2 sides of the wings. I was planning if it is possible to use the same filler to hone out a little valley to the metal along the cracks and refill with filler. possible?





also, on the parts I went down to bare metal it looked reddish brown






should I worry? would it have broke throught he filler after this long if it was going to be a problem?
thanks in advance guys, any help/advice highly appreciated