This is direct from West Systems instructions....
Thinning Epoxy
There are epoxy-based products specifically designed to penetrate and reinforce rotted
wood. Those products, basically an epoxy thinned with solvents, do a good job of pene-
trating 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 this results in similar compromises in strength and moisture resistance. Acetone and
lacquer thinner have been used to thin WEST SYSTEM Epoxy and duplicate these pene
trating epoxies with about the same effectiveness. If you choose to thin the epoxy, keep
in mind that the strength, especially compressive 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 (up to 120°F) of the project area with a
heat gun or heat lamp before applying epoxy. On contact with the warmed substrate, the
epoxy will thin out, penetrating cavities and pores, and will be drawn even deeper into
pores as the substrate cools. Although the working life of the epoxy will be considerably
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.