tripmind wrote:In a popular spot in my locale, there are some very prominent and very integral foot holds, and even some hand holds that are almost useless because of decades of shoe-rubber. Are there any reported methods of degreasing rubber off a foothold that is considered 'okay'?
This is a rather complex topic and none of the responses here fully address it. Different types of rock must be treated differently; something that is safe to do to granite will destroy limestone.
"Degreasing rubber" doesn't make any sense, and neither do several other suggestions here. A definition of terms is in order. A hold can have a combination of problems.
"Greasy": Usually caused by chalk saturated with skin-oils and humidity. Can be mitigated by brushing off excess chalk and/or waiting for dry conditions.
"Black foothold": particles of rubber fill-in the texture of the hold making it look black and reduces friction by preventing the rock's texture from biting as deeply into both skin and sole.
"Polish": The movement of shoes while weighted breaks off the tiny sharp points of the texture. Dirt on shoes acts as a polishing compound just like sandpaper.
On softer rocks
wire-brushing just smooths the texture further, and should never be used on sandstone or limestone. It
might be okay on granite.
Limestone is soft and chemically reactive. It dissolves in rain water (goutte d'eau, stalactites) and even very mild acids. It polishes easily, often to a golden "buttery" sheen.
By comparison granite is very hard and chemically inert. If often comes pre-polished by glaciers (Yosemite). Often footholds are made of feldspar crystals which stick out and, by nature, have very little texture to start with (Vedauwoo). Often the texture is caused by wind-blown sand (J-Tree, Vedauwoo) and not water; water has a tendency to polish (Fremont). Even polished, granite usually has more friction than limestone.
And of course, within the spectrum of rock between limestone and granite, each one is different. Even within a family there are significant differences; limestones differ widely, as do sandstones, granites, etc.
So the answer to the OP's question... as so often is true, is: It Depends. You asked "Is it okay?" so you know there could be issues.
Most of the time very little can be done without worsening the problem. But if you want to try you must first decide what is the root of the slipperiness. In the case of "prominent" feldspar crystals, you may find the holds were ALWAYS slippery.
If the sharp, thin edge has broken off, mechanical techniques may restore it, but you may also suffer the slings and arrows of other climbers.
If you decide to improve the hold, first TEST your approach (esp. chemicals) on some inconspicuous rock to develop skills and to see what happens. THEN poll MP for comments again.