Get some of Gilligan's Methylene Chloride, or Acetone, or what I use, Spotting Fluid. I spray or dab.
Interestingly, depending on the particular material, some do well from the front, some from the back.
It's not unusual to get a bit of a ghost, but that is not an issue when replacing with the same letter or shape.
Otherwise, I find that rubbing briskly with a clean rag dampened with the solvent helps.