I received this question in episode 5 of the Ask Brian Boggs Show:
When using epoxy for glue (like a Sam Maloof joint) how thick should it be?
I am assuming that you area talking about actual space in the joint rather than the viscosity of the epoxy. They are both important because in a Maloof type joint you have a great deal of area. It is going to be very helpful to thicken it with something like a wood powder or a thickener, which will help the epoxy stay in the joint so it won’t immediately seep out. If you have a fairly tight joint you don’t want to clamp it very tight. More importantly, you want to apply the epoxy and give it at least 10-15 minutes to thoroughly soak into the wood and then reapply again. The risk in not doing this is (after clamping the joint together) the wood can actually pull so much epoxy away from the joint through absorption that you won’t get a full strength bond. Even if it seems like it is bonded it might not be full strength. The viscosity is important, getting the wood to presoak is important. You also want to have a little bit of space because you do not want a totally tight joint with a lot of clamp pressure. However I found that you don’t need a visible gap between the two parts in order to have enough epoxy in there to react, especially if you allow the epoxy to soak into the wood first. You are going to have enough body of material there for the chemical reaction to take place. That is the concern a lot of people speak of with concern of epoxies. If you don’t have enough material present (unlike PVA glues or hide glue where you can have virtually a molecule thick glue line) epoxy does need more material available. Like I said, allowing it to soak into the wood pretty much gives you enough body. I know this because we use a 20-ton press in our laminations and we are getting the wood to contact itself.
In tests those bonds have proven to be stronger than the wood itself. You can have a nice looking joint along with a really good bond without having to use so much epoxy. You won’t have a big thick layer of glue between your joints or wooden parts.