Good to read you are looking at this with a slightly more positive attitude..
80 psi on large panels is way too much in my world. I stay at 45 psi with play sand, which leaves a texture of about 80 grit. Your surfaces appear to have a lot rougher texture than any I do, like the oxide is quite aggressive. Yes, the lower pressure and sand takes longer, but I can do this without warping. He is in a hurry to make profit...