In addition, I recently bought a Badger "air eraser" kit (just a
bottle with some grit and a tip) and I have hooked this up with my
grit booth. I find I can safely remove small amounts of lettering
with very fine control, and I've even started using it as a
weathering tool. For example, I use it to "whiten" freight car
lettering which I have hit a little too hard with grime. In another
case I had a box car with a primer coat of "galvanized metal" color
and an overcoat of box car red. I was able to remove some of the top
layer so the galvanized color showed through. Very cool!
If you want an even gentler abrasive for that fine partial removal, try
Bicarbonate Soda aka Baking Soda.
And you can do the blasting outside, which gives you a better view of the
work than through the window of the blasting cabinet.