Try a bit of spit and fine cloth..and rubbing..persist. ;~`)

by John Meeker , Wednesday, February 12, 2014, 13:01 (3943 days ago) @ Gila Jorge

if not too bad, it will do the trick. also finely powdered white chalk and spit and cloth -- all very carefully and frequent checks to make sure the blue is not being shaded or highlighted. I'd stick with spit and cloth.