I am using the 60 grit belts I got from machine Mart but I find the join in the belt comes round with quite a clunk when it hits whatever is being sanded. I changed the belt recently but the new belt was the same.
Geoff, the sander has been very good considering the length of time I've owned it, and used only for metal. I think it was only about 60 quid at the time and they are over 100 now. On the first one I bought, the motor burnt out in no time, but they changed it no questions asked and I ended up with a spare belt for the new one.
edit: Memory man strikes again, I've just been in the garage and found some more, 80 grit, belts that I didn't know I had. I'll stick one of those on and see what it's like. No doubt I bought these for wood when I got the scroll saw, then immediately forgot about them.