I've never painted rubber, but I assume oil based paint, possibly model paint would be the way to go. I'd just paint a block of wood and lay it across the letters, rather than painstakingly paint each letter.
Have you tried Googling it?
Acrylic is water based and brittle, so I wouldn't expect it to last long. I think I'd try spray painting a piece of wood and transferring it to the letters. Spray paint seems to stick to anything.
How do you think the printing press worked? It may not have been wood, but the concept is the same. Glue a piece of felt to the wood if that helps.
Oh, the letters are already white? Isn't there some type of cleaner on the market? I used to use a coarse brush on mine, but you have to go with the grain or it doesn't work very well.
Thread over.So I've used this paint pen from Dupli-color several times and it is a good product. The paint does hold up pretty well. It will, however, depend on what you're using it on. When I had my sports car (that would see plenty of cornering action) the tires would get more flexing movement and it would crack the paint a bit. Still, it would hold onto the tire. With my truck, that I drive normal, the lettering holds up much better.
This pen does require some patience though. You literally have to dab the paint on all the lettering. It doesn't just color in like a marker. You should be able to find these at your local Autozone or O'reilly's. They are like $8. If that doesn't work or up to your standards the next move (which was mentioned above already) is ordering a vinyl lettering kit. You can get them in any size and wording you want and they hold up pretty good.
