after the rim is off, take a good look at it. if there's lots of dirt like mine, then you definetly need to clean it (duh!!)
http://i192.photobucket.com/albums/z85/neptune_51/My%20Stuff/a0a7.jpg
as you can see mine are pretty bad
next you need to clean out the entire thing. use a cloth and the hose to get most of the visible dirt
http://i192.photobucket.com/albums/z...Stuff/4635.jpg
after cleaning the rim, start sanding with the 600 grit. go through the entire thing so that it's super smooth
http://i192.photobucket.com/albums/z...Stuff/8352.jpg
you can sorta see the places where i sanded in the picture...