An antique bike is one thing. I don't think I would touch one with anything but cleaner. A collectable bike is a little different. Unless it is in collectable condition, a new paint job probably would not detract from the value, if it's a quality job with replacement decals. I had my wife's Mercian repainted with metalic green Imron after adding the usual braze-ons. I am sure it is worth more now than in it's scratched up but original condition.