Hey guys couldn't find much on this either on this forum or anywhere on the web really...well not anything helpful. So last time i ordered belts they didn't have what i had been using in the 600 grit range so i split the order and got 2 different kinds. I went to use one the other day and i noticed the joint on the belt was real loud ya know thump, thump, thump, and then i noticed it was making horizontal "divit" (for lack of a better word) lines. A couple of other people confirmed that these lines were probilly from the joint as i suspected. So ok i figured its a learning experience and take it as a loss and never order those belts again. So on the last order the 400 grit belts i got the exact same kind as i have been using (with no problems before this). So yesterday i went and grabbed one of the new 400's (again same exact kind i have used before) and the same thing happend with the lines apearing. So to test it i grabbed one of the same belts but i had left over from a previous order. I used one of the older ones and it was completely fine. Then i noticed even tho they are exact same belt the new ones are slightly different color (just a lil bit darker). All the new ones did this old ones didnt i dont get it. And now i got a bunch of belts that i really cant use and i dont even know wich ones to order. Does this happen alot where the same brand of belts have different  outcomes?  Is there ANYTHING i can do to fix this problem? Any advice would be really appreciated i am completly stumped and dont knoow what to do but these lines are deffinitly not acceptable. Thanks ahead of time guys

  The "bump" as you go over the joint is pretty common with higher grits.  I want to say that the rotary platen that Beaumont Metal Works makes was designed to help address this issue.  I have never used it though so I can't speak from experience.

  Personally I don't grind much past 200g and if I do it is typically on a slack belt.  It has been my experience that it is much harder to get a clean finish with belts once you get to higher grits,   so I take it to 200-300g on the grinder then switch to hand sanding at one grit lower than I finished with on the grinder.  The only time I use higher grits is when sharpening or when satin finishing parts,  both with a slack belt.

Just a guess, but you're probably using a "J' weight belt which is extremely flexible. They work fine on a wheel, but not so well on a platen. I might suggest using Trizac gator belts.

The numbers don't correspond with what we associate with various grits. You'll have to look up the differences.

John