How body filler is spread really matters. It can effect pin holes and air pockets in the filler (which might show up through the paint), and doing it well can help ensure adhesion.
There are a few things to notice in the two pictures.
♦️The first, is that I am spreading the filler over 80 grit. I am very strict about this, every 40 grit scratch gets sanded out. The filler can go over finer than 80 grit as well, but 80 grit is the coarsest.
♦️The first pass is very thin (the first picture) and pressed in very good. This is to make sure filler is squeezed into any scratch or pin hole, and it allows the filler to adhere better. Otherwise the body filler is more just laying on the surface.
♦️The second pass(picture below) is where the filler is actually added.
♦️I make several passes to get as many air pockets squeegeed out of the filler as possible. The more you can spread it the better to work everything out of it. Try not to just spread the filler on and leave it after one pass with the spreader.
♦️Notice the angle of the spreader. It is pretty flat with the panel. The more parallel with the panel the spreader is, the better and smoother to filler will be.