Depending on the rubber mixture, you can dissolve the outermost layer, but it will evaporate again and it's never a lasting solution, since
the problem is almost always that some (solvent-based) ingredients within the rubber mixture simply evaporates or certain chemicals seek to
the surface and makes it hard with loss of friction as the result.
In many Beocords, you can grind off the hard surface using sandpaper, the surface will first turn white and hard and underneath it
will be soft and rubbery. But what has gone has gone, and it will never be like new again.
Coatings will not last and you cannot make these rubber mixtures absorb "something" that wont ruin it.
Only proper solution is a replacement.
Many Beocord idler wheel rubber parts are available now (from me). Made from a fairly new material, that doesn't dry out, doesn't lose friction and
with practically no "memory effect" (it will fall back and not stay deformed if forced out of shape, even for a considerable amount of time).
New Beogram idler wheels - very soon.
Martin