Who's the diff daddy??

[Pops]

Can someone give me some tips on building a diff that lasts more than one event without becoming coarse and notchy? How often do you guys rebuild? Which diff balls are best..I use the std carbide ones, sand diff rings on a flat granite bed with 1000 grade wet/dry using a spot of wd40.When first assembled the diff is really smooth and i get that oooh feeling when i turn it!!! After a qualifier or 2 its slightly course and after finals it feels nasty so i do it all again(2 x6's and a b44!) Heeelp please
Thanks, Dave