Originally Posted by
Duragrouch
When they said machined sidewalls, I think (not certain) they meant ones with intentional machining grooves running cross or diagonal to pad direction, sorta like from a slightly inclined fly-cutter or insert machining head. I think I've seen rims like that, and also with some sort of ceramic coating (cerakote, like firearms?) I could really use that, my rims are too polished smooth, but also concaved and thus thinning there, I don't want to remove any more metal, even with a small flexible sanding disc.
Some rims are skimmed on the braking surface so the coating doesn't interfere with braking, and to make the rim surfaces more precisely parallel, some rims are lightly grooved which increases brake contact area, but not as much as some rims in the 1970s/80s which had the grooves extruded into them. Some trials riders scuff their rims with a coarse grinding disc to increase holding power rather than stopping power.
Ceramic coating makes the treated rim very resistant to abrasion and wet braking is reportedly improved with some processes but it needs special block compounds which tend to wear rapidly, and take a while to bed in. Historically the coating has a tendency to crack and flake after some use, but rim companies have been working on this for decades and newer processes are claimed to be better. Ryde used to offer a process that embedded tungsten carbide particles in the rim, but they seem to have dropped that - apparently mediocre brake performance when wet, as rims polished with wear, was a problem.
Rim brakes make rims a wear item, replacing them is more hassle than swapping out disc rotors, but we lived with that for a century before modern disc brakes.