Harry,
I use Flitz metal polish to remove old gunk on bows (says it is good for polishing fiberglass on the tube). It is mildly abrasive, so I don't scrub hard in the area of any silkscreens, decals, or lettering.
I have read that BowDoc makes some really great stuff, but I have yet to hook up with him to buy some. One of these days we will end up at the same shoot somewhere, as he is in Seattle and I am in Oregon.
Ron