Yup, it’s a real thing.
Not what I was really talking about, though. In this case, it has to do with some of the issues that come up with working with wood, especially reclaimed wood that hasn’t always been treated nicely. There are frequently splits or nail holes or knot holes that visually spoil an otherwise sound board. In the case below, the boxed heart timber I was using had split along one of the faces, and I wanted to stablise it for reuse.
(An extended aside here: Boxed heart timber is inherently unstable. As you may know, timber wants to shrink across the grain. Since the grain is across every face on a boxed heart beam, it’s trying to shrink on all four sides at the same time. Something has to give! Invariably, at least one of the sides will have split from the stress. If you can, strive to find FOHC [Free Of Heart Center] timber if possible.)
So what was I to do? The answer could involve planing a groove (possibly putting a saw kerf in there if it was a small split) down to the pith and then gluing a plug in there. If it were a more cohesive split, I might have done that. But since it was a stringy, disjointed split, I chose to use tinted epoxy to fill it. The pigment (just regular artist’s pigment) isn’t structurally necessary, but the yellowish cast of the stock epoxy makes it look like your wood has solidified snot in it, rather than just a dark patch of grain. ALWAYS PICK BLACK for this application. You can make it various browns, but they’re never exactly right (especially over time), and they make it look horrid.
So, this is one of those times where you must have faith. Once you goop this stuff on there, it will look like you’ve taken the Exxon Valdez method for woodworking. Don’t fret. It will be fine. If your splits go out an end, make a dam with some painter’s tape so it won’t run out. Keep an eye on it for an hour or so, because the epoxy will sink into the bottom of the crack and you may need to top it off.
Once it’s had a chance to cure thoroughly, the excess will plane right off, and voilà! The splits are now filled all the way to the bottom of the crack, and you can use the chunk of timber now without reservations as to its stability. Aesthetics are also improved since everything is in the same plane, and also won’t collect dust like a crack would. Under a coat of finish especially, it will blend right in.