I'm painting a 4 X 8 Sign going on the outside of a building. I'm purchasing preprimed MDO. My question is, what type of paint should I use for the background color and the lettering? Flat, exterior, high gloss, types?? Also, I was thinking of rolling some polyurethane on it when I was done, even though I think I don't need to, just to better protect it...yes? Any help would be appreciated. Thank you.