Mike Beckmann
Owner/Chemist, Bonstone Materials Corporation

There seems to be a trend lately towards using more natural stone outdoors: limestones, marbles, and granites are being used for everything from lawn ornaments to fountains to lawn furniture, and even outdoor kitchens and fireplaces.   
People are very familiar with the use of such stones inside, in a controlled environment, but with the variable climatic conditions of outdoor use (rain, snow, heat, cold, wind abrasion, etc), it’s a whole new ballgame!  What works inside may not work so well outside.

So, let’s look at the use of materials commonly used to fabricate, install, and repair natural stone used in outdoor applications. There are several varieties of adhesives commonly used in the stone fabricating and restoration industry, including polyesters, epoxies, and acrylics. There are, of course, other products used, but these are the most common.  If you’ll recall from a previous article, each of these technologies has its own set of “plusses and minuses,”  so let’s review a few of them here, in the context of using them outdoors.

POLYESTER: Some of the “pluses” are fast cure speed, relatively lower cost than the other technologies, and familiarity/user friendliness (they’ve been around for a long time and people know how to use them). One of the main weaknesses of this chemistry, however, is the high amount of shrinkage that occurs during the curing reaction, and that continues (albeit slowly) throughout its useful lifetime.

This shrinkage has detrimental effects:  when used as an adhesive, shrinkage causes poor wetting of the substrate, and “pullback” from the substrate,  which results in  poor adhesion; and when used as a repair material, the shrinkage causes the polyester to “shrink” away from the surface, causing a divot, which requires a second application to fill flush with the surface.

Another weakness is the relative sensitivity to moisture, both during and after cure. A third problem is the poor UV-stability, resulting in yellowing when exposed to sunlight. For these reasons, polyesters are not recommended for outdoor usage. Use one of the other technologies instead.

ACRYLIC:  These materials are chemically similar to polyesters, in that their cure mechanism is basically the same—so they have the same basic characteristics of polyesters:  fast cure, but also with the shrinkage and accompanying problems. Acrylics, however, do have two advantages over polyesters:  1) their adhesion and strength is slightly  better;  and 2) they are UV-stable, so do not yellow if exposed to direct sunlight. They are OK for use outdoors, especially in areas when UV-stability is required (ie: exposed seams),  but if you really need the best long-term strength and durability, use an epoxy.

EPOXY:  Here is the ultimate outdoor adhesive. Epoxies are known for their tenacious adhesion, and this “hang on like a mad pit bull” is what you need when you are building fountains, benches, barbecue pits, countertops, etc., in the aggressive outdoor environment. And their moisture tolerance and internal strength contribute to their unique ability to “weather” the extremes of climate exposure (moisture, temperature, abrasion, etc).

The only weakness of the epoxy systems is the tendency to yellow when exposed to the UV rays of the sun.  It should be noted, however, that this yellowing is a surface aesthetic issue only, and it in no way affects the integrity, strength, or performance of the epoxy adhesive.

So, to sum up: 
• If you have a really aggressive application outdoors, use the epoxy. 
• If UV-stability of the joint line is of paramount concern, choose the acrylic (or another UV-stable material).
• It is recommended to never use a polyester outdoors—unless you like dealing with failures!