Author: Tom Cordova of The Rock Blog
The most important issue you need to know about using granite outdoors is that Ultraviolet rays from the sun breakdown polyester epoxy resin.
Basically, there are two places that polyester resin is used during granite fabrication. First, the overseas factory sometimes fills the face of the granite with this epoxy before polishing to fill all the small fissures. If this is not done, the small deep pinholes and pockmarks will be visible after the polishing is done. This is a common practice and an excellent practice for granite that will be used for kitchens or other indoor use because it adds protection to the granite as well.
However, in cases such as Class D granite, known for having many fissures and pits, there is a substantial amount of resin fill that is applied. While the Class D granites are fine for interior use, I strongly recommend you do not use them where they will be in direct contract with sunlight. The sun's rays will breakdown the surface resin, first turning the resin yellow, and then literally fracturing the resin crystals themselves.
This is another reason why Engineered Stone, like Caesarstone, is not recommended for exterior use either.
The good thing about the above situation is that this does not cause total failure of the granite. In fact, I know of a customer that had this happened and he actually liked the 'antique patina' that the decomposed resin created on the granite!
The most important thing to remember is DO NOT use laminated edges outdoors. Use solid granite only. Whether it be 3/4" or 1 1/4" granite, do not add laminate edges. These will fall off because the ultraviolet light from the sun will breakdown the polyester resin in the seam and cause it to separate. It could take 1 year, or maybe even two, but it will happen.
Some fabricators say they will use 100% two-part epoxy made for outdoor use. However, this epoxy is not fluid enough to be used in the seam as a joint binder. It can be put behind the lamination for additional support but the color polyester resin will still be used in the seam.
Just don't do it! This is an easy disaster to avoid.