I used durarock for the backer. I used primer throughout the whole process. I have done some small sample pieces with this and checked for moisture penetration. None. The primer is essentially skinning over creating a moisture barrier. After coloring I used a 2 part urethane applying 2 coats within a 12 hour window. Hope this helps.
Eric its no problem at all to get it 2" with the sprayer I was going to call you this evening. I am still here with Thom in Nebraska and we are adding on to the studio. I hope this helps, let me know if you need anything else I will be glad to help.