For those of you looking to find a VERY tough exterior coating,( but don't want to spray life threatening catalyzed materials using external air supplies...) there is another option. System 3 makes epoxies, and coating for boat builders. They now have a WATER BASED LINEAR POLYURETHANE coating for boats which should give better protection, with less hazardous application, than many people are now using !
If there are any doubts about the abilities of water based polys, take a look at many of the world's best auto manufacturers, they have been using similar stuff for the last few years, and current legislation will have everyone in the industry shooting water based coatings in the next five years...The product offers various finishes, 12 "stock"colors (with custom blending available), and a "cross linking" catalyst for those who need the extra shell of protection.
It also makes sense to consider your OWN future when doing topcoats, so if you can utilize a product which is less hazardous, why not use it?
I seriously doubt that most signs, and projects will get the constant punishment of a boat hull , that is immersed in water ( which is also usually "blended" with various petro chemical contaminants...), baked in reflected sunlight, and pounded into waves and wind on a regular basis. (If your customers DO have signs that get this kind of abuse you might want to post some pix here...)
The link to this product is;
http://www.systemthree.com/p_wr_lpu.asp
I know that Jamestown Distributors stocks it, but I'm sure if you are really interested, a visit to Google will find you a local source.