Cork belts are "finishing" belts..... used kinda like scotchbrite belts. I keep 600 grit cork belts in the shop for specific finishes. They are best when "broken in".....which in reality means take a piece of scrap steel, and lay into them HARD..... when they quit cutting on the steel, they are ready to use.
Cork belts must be broken in. I use a piece of black iron pipe. Round bar works well and I've heard of guys using brick. I think Ed's flat bar will also work. The recommendation when I bought was 5 -10 minutes break in. I go to a X15 then to a 400 cork.