Wood rot can develop anywhere. The best fix is replacement, but if this isn’t possible, epoxy fillers are an excellent alternative. They hold paint well, and most can be readily sawn, sanded, planed, and drilled. Fillers range in consistency from syrupy thin to stiff pastes; the latter allow you to fill and shape an epoxy to match a sharp edge or a complex profile.
The trend in epoxy fillers is focused on convenience. Resin-to-hardener proportions are usually 1:1 ratios (sometimes 2:1), which are easy to measure during field use. Some fillers are formulated to take hot or cold temperatures into account so that curing times are neither prolonged nor too fast to be useful.
In exterior applications, the wood surrounding an epoxy repair will continue to expand and contract with changes in humidity; consequently, the more flexible the epoxy, the better the long-term result. A rock-hard filler will eventually break its bond with the surrounding wood, allowing water to enter around the repair.
Finally, even though epoxy is expensive, familiarity can sometimes make it seem like the answer for every problem—it’s not. (Don’t use it for structural components like joinery, for example, and you don’t need $150 worth of epoxy when you can splice in a $10 piece of wood.) Used judiciously, epoxy will be one of many important tools in your old house maintenance toolbox.
Head to Head Test
Mixed epoxies give off heat as the resin and hardener polymerize, and since heat accelerates curing, they can cure quickly, especially on hot days. To prevent this, keep unmixed epoxy in a cool place, and put the mixed components on ice if possible. At the very least, spread the mixed epoxy on a flat surface (such as a waxed paper plate) so it can dissipate heat more rapidly than if contained in a cup.
–Ray Tschoepe, Director of Conservation, Fairmount Park Historic Preservation Trust