iam curious ive been using lacauer primer to showdow coat car as i block sand car looking for high/low spots. am i gonna have any problems with these layer shrinking under the filler. and also should i sand all of them of and try and build back up with only poly filler. and can you get urethane primer in a spray can without buying gun

Using dark lacquer as a misted guide coat should cause no problem. When you detect low areas, you then lay on your build primer and another guide coat. I assume build is urethane.
as for U primer in rattle cans. I am not aware of any.

just do what i do...

borrow a gun

You'll be fine as long as your only misting it on. You'll be sanding most of it off anyway. I prefer black over a light colored primer, and spray bomb is fine. As for the primer builder, I prefer a catalized primer (uses a hardener).