I'm working on making my own 4'x3' sign to replace an older sign I had someone else make years ago. They used Lusterboard/SignPly and I'm using the same material (it has a white aluminum face). I won't be using edge caps on this sign. The company that made my sign a long time ago also didn't use the edge caps but just used a sealant. It held up for about 7 yrs but now the plywood has completely rotted away. My question is what kind of sealant should I use?