An easy, simple way to paint the tops of cast letters is to use a cork.....
A local hardware store in this area sells mixed size bags of small (varying from 1/4 to 1/2" dia at the small end) stopper corks, and for surprisingly cheaply.
I've used these corks for years and years as masking plugs for the bolt or other holes when painting machinery, as they are cheap and convenient for the purpose.
A small, clean new cork works well for applying paint to letter tops. I use a good quality enamel for the paint, and, after stirring the paint well, put a teaspoonful of paint on a clean surface, like the bottom of a small tin.....a tunafish tin is about right.
By just touching the cork to the paint, enough paint will adhere to the cork that the paint will transfer to the cast letter by "patting" the letter with the cork.
Depending on the size of the letters, the paint on the cork will do from one to three or four letters before it "runs dry", so to speak.
Let the paint dry for a few hours time, and repeat the process....the second coat brings up the "depth" of paint on the letters.
A good way to repair damaged or mis-cast letters, particularly if the part can be glass-blasted clean, is to use "Devcon Plastic Steel" epoxy, or equivalent. This material has to be left overnight to cure sufficiently to work. Running over the lettering with a file held "lengthwise" of the lettering brings the built-up letter tops flush with the existing letters, and a tiny tapered rotary file in a die grinder will correct the outline. A careful touch here and there with a little triangular scraper tip will blend in the radii where needed.
Refinishing machine lettering this way is really quick and easy, once you've gotten in some practise, and the finished job will look nice, if you are careful.
cheers
Carla