ORIGINAL: da Rock
To thin epoxy, do not mix in solvents such as acetone or alcohol.
Acetone is epoxy thinner. It is sold as such. Read the label on the next can you see. Alcohol was not created as an epoxy thinner, nor is it the recommended thinner for epoxy. Acetone is.
Acetone was not created as an epoxy thinner either.
Heat works better because it reduces the risk of the epoxy not curing properly.
Acetone is epoxy thinner and while it affects the cure time, the epoxy does cure properly.
It doesn't always cure properly. Also, how handy is it to keep acetone on hand, at $5-6 per quart, for the infrequent occasions that one would use it?
Also, studies have shown that using a solvent causes the epoxy to be somewhat porous when it cures.
Studies have shown that water and alcohol not only cause porosity, but some is trapped within the curing resin and weaken the result measurably. Drug store rubbing alcohol contains water. It's the worst "thinner" you could choose for epoxy.
Studies have shown that adding ANY thinner to epoxy causes porosity and weakens the epoxy. Here's an example directly from West Systems, a well known maker of epoxy:
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.
If that's not enough information, here's more:
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.
Adding a volatile solvent extends the pot life and cure time of epoxy and jeopardizes the reliability and predictability of cure.
Adding solvents, especially acetone, alters the color of the cured epoxy. While the effects are not immediate, adding acetone to epoxy causes the color to change from slightly amber to very dark amber.
In the R/C hobby, we use epoxy for strength, as a chemical barrier, or both. Since thinners compromise epoxy's ability to perform either role, after reading this, and the studies they did to come to the above conclusion, for me it was a no-brainer to stop thinning epoxy with solvents.