To be honest I havent done any yet, was planning on it this weekend, but have seen some folks that do it with good success. The 'premium' vinyl is supposed to be sun and weather resistant, so whether youd need to clear coat it is up to you I'd suspect. Another option is to simply use the vinyl cutouts as a mask for painting. You could actually transfer the letters and the background with it, then strip the letters from it, and spray on top of a powder coated box.
Available colors from silhouette are:
Black, White , Brown, Dark Red, Dark Pink, Leaf green, Silver, Turquoise, Straberry, Sand, Blue, Navy, Orange, Pink, Frosted glass, gold, purple.
As for clear, maybe the frosted glass would work, but i've never used it.
There is also some chalkboard vinyl i could size for various boxes so you can just write on them yourself, make marks for settings, etc...
I would be able to make any labels you wanted, but they would depend on my availability.