For those of you that end up here looking for ideas...here's the product we ended up chosing for the clear coat. What we found as the result of some calls to various vendors was that there are very few clear coats for acrylic paint that have an exterior application. Almost all are interior only. Now we don't know for sure if there is a big difference, but we do know this stuff cost a pretty penny! The vendor we use, Monarch in Chevy Chase, assured us however that the price reflected the specialized application and that it is a quality product. We're happy with how it covered without distorting our artwork, now we just need to see how it holds up against the Maryland weather!