There are epoxy-based products specifically designed to penetrate and reinforce rotted wood. These products, basically an epoxy thinned with solvents, do a good job of penetrating wood. But the solvents compromise the strength and moisture barrier properties of the epoxy. WEST SYSTEM epoxy can be thinned with solvents for greater penetration, but not without the same compromises in strength and moisture resistance. Acetone, toluene or MEK have been used to thin WEST SYSTEM epoxy and duplicate these penetrating epoxies with about the same effectiveness. If you chose to thin the epoxy, keep in mind that the strength and moisture protection of the epoxy are lost in proportion to the amount of solvent added.

There is a better solution to get good penetration without losing strength or moisture resistance. We recommend moderate heating of the repair area and the epoxy with a heat gun or heat lamp. The epoxy will have a lower viscosity and penetrate more deeply when it is warmed and contacts the warmed wood cavities and pores. Although the working life of the epoxy will be considerable shortened, slower hardeners (206, 207, 209) will have a longer working life and should penetrate more than 205 Hardener before they begin to gel. When the epoxy cures it will retain all of its strength and effectiveness as a moisture barrier, which we feel more than offsets any advantages gained by adding solvents to the epoxy.

Adding 5% lacquer thinner to epoxy reduces the epoxy’s compressive strength by 35%—a big hit in the mechanical properties of WEST SYSTEM epoxy (Figure 5). The addition of more than 5% solvent results in an excessively flexible cured material. Thinning epoxy with solvent causes enough loss of strength that we (and most other reputable epoxy formulators) cannot recommend using it as a structural adhesive.
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Adding volatile solvent may cause shrinkage of the cured epoxy. Applying thinned epoxy in large, confined areas (like consolidating a large pocket of rotted wood) is likely to trap some of the solvent. In thick applications, the epoxy cures very quickly and not all of the solvent has time to evaporate before the epoxy hardens. Over time, the solvent works its way out and as this happens, the cured epoxy shrinks and in many instances cracks. Shrinkage also causes print through. You may have a surface sanded smooth only to have the resin shrink. This shrinkage often reveals the texture of the substrate. Shrinkage can continue to be a problem until all the trapped solvent works its way out of the cured epoxy.
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Does thinning epoxy make sense? In some situations, thinning is appropriate. In others, it is not. We feel that in most circumstances using heat to thin epoxy is preferred to using solvents. As long as the epoxy does not overheat during cure, the full physical characteristics of the cured epoxy remain. Adding solvent is a quick, simple method of thinning epoxy, but the strength and moisture resistance of the cured epoxy are significantly reduced.