Most epoxy resin manufacturers have spent a lot of time and money refining their formulas. They don't usually recommend thinning their products with solvents because solvents can change the chemistry of the epoxy and adversely affect its strength properties, cure time, and proper curing.
Use a low-viscosity epoxy system (recommended): For example, if you're working on a project that calls for thin epoxy for repairing wood rot or low-viscosity epoxy for wetting out fiberglass cloth, there are epoxy systems formulated specifically for these purposes. Examples are TotalBoat Penetrating Epoxy (for wood rot) and MAS 2:1 Low Viscosity Epoxy Resin or TotalBoat High Performance 2:1 Epoxy Resin (for easy wet out of fiberglass cloth). These products ensure proper cure times and cured properties.
Thin your epoxy with heat (recommended): Heat is the best way to thin epoxy because it makes the epoxy more workable, without changing the chemical composition. There are two ways to heat epoxy:
- Heat the separate resin and hardener components before mixing, mix the heated components thoroughly, and apply. But be prepared—heating the components will make the resin cure faster. To slow down the cure, use an extra slow hardener. Epoxy systems with extra slow hardeners include West System 105 Epoxy Resin combined with their 209 Extra Slow Hardener and TotalBoat 5:1 Epoxy Resin combined with their Tropical Extra Slow Hardener.
- If you're coating wood, heat the wood surface and apply the room-temperature epoxy mixture to the heated wood. While the wood is cooling, the epoxy will be drawn more deeply into the wood, creating a moisture-proof barrier.
Thin your epoxy with solvents (not recommended!): Unless the solvent and the thinning percentage are specified by the manufacturer, we don't recommend thinning epoxy with any type of solvent, because solvents can drastically change the properties of the epoxy. For example, adding a relatively small amount of lacquer thinner to some epoxies can reduce compressive strength to such an extent that the epoxy can't be used as a structural adhesive.
Here are some other reasons not to add solvents to epoxy. Adding certain solvents can:
- Extend curing time
- Prevent curing at all
- Create resin-starved fiberglass fabric when wetting out vertical surfaces
- Damage some surfaces (always test solvent on substrate before thinning)
- Change the color of cured epoxy
- Make the epoxy dangerously flammable, until cured
CAUTION: If you decide to use a solvent to thin your mixed epoxy, be sure to understand how doing so will affect the outcome for your particular project.