2" X 72" COMPACT GRAIN BELT 600 GRIT. Compact Grain/Cluster metal working belts can last three to four times longer than AO belts. They leave a satin, brushed look on metal. Knife makers and sharpeners love them. Due to the chunks of grain, their appearance is much rougher than their stated GRIT.. It is advisable to break in each belt for 30 seconds with a piece of metal. Thereafter the GRIT. remains consistent. When the GRIT. on these durable belts is consumed, many of our customers add polishing compounds such as jeweler's rouge for extended use.