Cgav34,
Why not consider Corian as an alternative in this case. I do 99% of my work in Corian type products, and it holds up quite well outdoors. Paint the letters, sand, then coat with a rattle can spray lacquer and your good to go. My address sign has been up since probably 2010 and looks as good as the day I made it, just a thought... you can get sink cut outs from kitchen and bath remodelers. A lot of times just asking will get you a piece of Corian.
Gary
3D Pro Ver. 3.103 Build 14, STL & DXF Importer, Rotary, Conforming Vectors, Centerline- Intel(R) Core(TM) 2 Duo CPU T6500 @ 2.10GHz 4GB Ram 64 bit Operating System using Windows 7