I seem unable to get a fretboard level. I bought a radius block from StewMac but now I can't keep the sand paper from gouging grooves into the stain I applied.
Are you using the Wet or Dry automotive sandpaper? If it builds up, you need to use it wet. Sometimes a little dish soap in the water will help keep the paper from building up. The type of finish you are sanding, how hard or soft it is, will affect the buildup on the paper.