FrenchFit 

Have a vintage MTB rim that got ridden in the sand, I'm guessing. It's sounds terrible braking, (with fresh pads), but visually it's not badly worn at all. ( I want to keep the rim, but do the equivalent of machineing the surface).

How to?: what grit/type of sandpaper, spin the rim?, use a block?, pointers?

fietsbob

put in new brake shoes to get rid of the embedded grit., then ride the bike ..
the rim will get thin soon enough, I wouldn't speed that up
removing any more metal.
Clean the bike, particularly after riding in the dirt. soap and water.

FrenchFit 

Done, but it's not enough. I need to hasten the process, it think the rim has fine grooves, and that will kill the pads. If the above process would work, you'd never need to replace car rotors, just change the pads.

canopus 

You should never need to replace your car rotors. If you do you ignored basic maintenance... Or wrecked

What sound has you concerned? Is it grinding or just noisy? Squealing? Fine scratches such as sand won't cause it to be noisy. Check them again for any extreme gouges. I'm from the no sanding school of brake noise repair. The only time I've had to sand a rim is to smooth out a gouge in the side.

Yan 

Wash and ride. Any scratches will soon disappear. Sanding and filing is for major gouges only. If you really want to sand the rim, use some emery cloth, available at any good hardware store.

FrenchFit 

Never need to change your car rotors? You've never owned a BMW!

I jumped in: Bronze wool w/Semichrome, Simple Clean and buffed out. OMG, my pads have died and gone to heaven; noiseless, perfect brakes...

I appreciate the posters thoughts nonetheless, but if anyone is thinking their rims needs a little TLC I can attest this fix worked perfectly.

BHOFM

The Scotch Brite pads work well also.

yannisg

I use Scotch Brite with auto scratch removal paste successfully to clean the rim braking surface.

YannisG

MTBerJim

I don't know where you got this idea from, but it's just not true. When you've worn out a set of brake pads, it's recommended to resurface the brake rotors. Once they are less than the manufacturer specifications they need to be replaced. I've changed 100's if not 1000's of brake pads in the 10 years I worked as and auto tech, the only time I had a problem was, when against our recommendation the owner declined NOT to have the rotors cut/replaced. If you just slap on new pads it's almost guarantied that they would get braking pulse in the steering.

MTBerJim

+2 on the Scotch Brite pads

fietsbob

I have several Mavic EX 721 rims, black finish,
machined brake track was grooved out of the box.
it's smooth now, just riding along..

another rim version CD hard anodized, came on the bike
braking was grabby, noisy, so I swapped in a more abrasive brake pad.

Brake : Magura Hydrostop hydraulic rim brake, their Green is a more abrasive pad
a couple weeks of downhill wet weather braking wore thru the anodize surface,
then I swapped them back to the less abrasive original black pad and the braking has been fine.