I make some trail signs for my friends property. I built them out of oak, with letters engraved on my CNC machine.
I tried a couple of methods for painting the signs - masking the flat face of the wood, hand painting the engravings with a tiny paintbrush, and finally settled on filling each letter with paint using a syringe.
After painting I gave each sign a couple of coats of polyurethane.
I drilled a hole in the side of each sign, and used a metal tap to cut threads into the sign. Then I used a hanger bolt to install the signs to a 4x4 post - the bolt has lag bolt threads on one side which hold it to the post, then I could thread on the sign using the cut threads on the other side of the bolt.
This worked fairly well, but it was very important to get the hole into the 4x4 and into the sign very straight - otherwise the sign would install on an angle and bind against the post before it was fully snug.