Painite is metallic only - metal rich has its uses but Painite isn't one of them.
Using the RES centre and planet as navigational aids, make a note of which three asteroids have Painite. They will also have Painite next time, and every other time [1]. So you know where three are and can get them with three limpets next time. That leaves the rest of your prospectors to discover some more. Repeat, and eventually you know where twenty or thirty are ... and you just hoover up the Painite as efficiently as possible, sell it, and repeat.
The skill is in recognising the asteroids again the second time - belt clusters are much easier to start practising on, though you'll only get Palladium or Platinum out of them at best. Get it right, make yourself a detailed map, and riches that if it was a mission there would have been calls to nerf it can be yours...
[1] They restock every two hours, so you'll need more than three in total to make a career of this.