Need some paint advice

I think I've got the whole process down pretty well as far as spraying the paint goes. I'm just not sure on the mixing process. What I've been doing is mixing the paint 1:1 with thinner/reducer, and 4:1 for the clear and hardener. Everything seems to turn out just fine, but I've been noticing some problems with the durability of the paint.

On to my questions! I painted my brothers car about 2 years ago now, and I was repainting his front bumper a couple weeks ago due to a run in with a ditch or two. I noticed that the area behind the front tires looks like it's been sandblasted down to the original color. Is this just from using cheap paint? Poor prep work? Are there more additives or a different process I should be using? I used a gray primer, sanded it smooth, then put on a couple coats of color, then a couple coats of clear, followed by wet sanding and buffing. I'll be putting a new rear bumper on my car (as soon as I find one) and I don't want the same thing to happen to it.

Some rock chips are too be expected, but this looks like someone literally took a sandblaster to it. That whole area is back down to the factory paint. Plus the rest of the paint chips easily and it's fairly easy to make the chips larger, like with a pressure washer.

I have been mixing and matching brands on a few things I painted, but not on this car. Everything came from Napa...Crossfire I think it's called. I'm using PPG Deltron 2000 paint on my car...So I should get the matching primer and clear for that instead using the Napa stuff that we have left over?