Caveat - I'm not a fiddler, but I have seen the "recommendation" that a bow that won't "take" rosin can be wiped down with a very small bit of alcohol on a soft cloth - on the hair only. The solvent will soften the rosin, so that rubbing gently takes off the "clumps," and the small amount left on the hair will dissolve some of the rosin and "spread" it on the hair as it evaporates, to provide a little "tooth" to pick up subsequent applications.

One luthier(?) was quoted in a recent magazine article (which unfortunately I didn't save) as saying that he "primes" bows this way when he does a re-hair - although I got the impression he used a lot more synthetic fiber "hair" on student bows than real hair on good bows.

Note that the varnish on most good - and especially most good old - fiddles is little more than rosin dissolved in alcohol or turpentine. (Plus, of course a few "magic" ingredients known only to the maker.) Anything that will dissolve, or even soften, the rosin should be considered a "hazardous substance" anywhere remotely near a fine instrument. (Based on personal experience with a few "garage sale" $10 instruments.)

John