Hi George
Reading your description reminds me just why I gave up restoring my signs in the 70's. If you want to get a good finish, the only way is long painstaking hours of careful work. The filling alone takes so much time!
However one thing we used to do, which you may care to try, was to spray the signs with cellulose paints using a spray gun and compressor, with an airbrush for fine work . Again preparation is all, in that the atmosphere had to be totally dust free. We used to use Frisk masking film and a scalpel to cut the shapes, then spray evenly across the sign in long strokes making sure to turn and return, off the sign, otherwise you got pools of stronger colour. You must allow time for each coat to dry . The other point to remember is to wear a mask and to make sure the room is propery ventilated.Of course anything else in the room gets a light coating of colour but we were art students then and had studios to play in.
We also used cans of car paint spray, which must be used in a similar fashion. Ideally if you know someone who sprays cars they will have the right set up and look at just how shiny car paint is!
The biggest sign I did was a Mitchell and Butlers leaping deer which was 6ft by 4ft, I must have put two square foot back in and then added the lettering - happy days!
Best Regards for a job well done
Richard