Having read through this thread I've noticed no one has mentioned specific hardners for gelatin sizing. I would recommend glutaralhyde. I've had great results with it. Working on unsized paper will almost certainly cause staining. The only gum printer that I know that doesn't size and doesn't get staining is Sam Wang.
Originally Posted by argus
Single coat gums are all but impossible to produce with any sense of depth and pleasing tonality. Be prepared to do multiple coats as Lukas has suggested.
Pre-shrinking is also an absolute must for proper registration for multiple coat gum prints. You can register by eye by placing the coated paper on a light box and then laying the negative on the paper. Using a sheet of glass on top of the negative will allow you to press the negative/paper sandwhich flat to get good registration as a dried gum print will be quite curly. Once the negative is registered, tape it down carefully with low adhesion transparent tape.
Oh yeah, printing by sun will drive you crazy trying to get consistent results, a better plan is to buy or make a UV exposure unit.