In case you don't like the reproduction boulder set (too shiny, too dark), here is my process to be closer to factory appearance:
Red Scotch-brite pad (quarter sheet is enough for full set)
Step 1: Remove shiny surface
Cut small pieces from Scotch-Brite. Remove the coat with circular movements.
Step 2: rinse the mountain with water (to remove dry coat particles)
Step 3: Remove excess paint
Wh2o_before (resized).JPGwh2o_scotch2 (resized).JPGwh2o_scotch3 (resized).JPGwh2o_IPA (resized).JPGWh2o_after (resized).JPG
Use toothbrush and Isopropyl Alcohol. Of course the more you apply/brush, the more paint you remove. Factory mountain has very few brown paint on it.
I kept a bit more to reach a realistic look.