Thanks for the questions and suggestions.
The cut outs will be painted black and the face of the letters and artwork will be painted white. The rest of the boards will only have clear coating applied. Helmsman has come out with an exterior water based finish that will not yellow the white paint......or so they say. I'm gonna give this a try and see how it works.
The sign's existing framework is in great shape......each sign's frame has two columns of bricks with 2 each 2X4 cross members extending from column to column. Each exiting sign has the mounting screws run through the face of the sign into the 2X4's.....this is NOT the way I plan to mount the signs!!!!
My plan is to bolt the signs to the 2X4's from behind......nothing going through the front of the signs. I will install brass threaded inserts with epoxy into the back of the signs, then run bolts through the 2X4's and into the back of the signs.......5 bolts per 2X4 board.....each 1" deep into the sign backs......for a total of 10 bolts holding each sign in place. Then, when the signs need to be repaired, I unbolt them, take them to the shop, sand the faces, repaint, reseal, then bolt them back in place. Since each cut out is about 1/2" deep, I can sand and refinish them several time be fore they need to be replaced. Hopefully he will be retired by then.
I will use brass inserts and stainless steel bolts and washers........need to avoid rust here on the Gulf Coast. The sign blanks are 1-3/4" thick. So with 1/2" deep cut outs, I don't have to worry about blowing out any of the mounting holes......plenty of material left between the bottom of the cut outs and the bottom of the bolt holes.