I am not getting your question. You have said that the lettering is 1.2 cm tall. Do you mean that you have engraved the background so that the letters stand out tall, or that you have carved the letters 1.2 cm deep into the wood?
In the first scenario, I think I will be equally as dumbfounded as you are. However, in the second scenario, I would simply paint the whole thing grey, then cover it with a newspaper or something which will cover everything but the lettering and prevent the grey color from contaminating it; then finally I would spray everything over with the grey paint.
Either that or I am totally off mark.