You've finished a breadboard end, or the cut end of a shelf, or the top of a leg, and watched the finish disappear into it. The face grain a foot away is building a sheen. The end grain is still drinking and still dull. So you do the obvious thing. You put on another coat. Then another. Three coats later the end has more or less caught up, and you've poured most of a small can into a patch the size of your palm.
It works. It's also the slow, expensive way to do something a single coat could have done, and it rarely gets explained.
Here's what end grain actually is. Cut a board across the grain and you've sliced through thousands of long cells lengthwise, so their open ends face straight up at you. It's a bundle of tubes with the lids off. Brush finish onto that and the liquid doesn't sit on the surface, it wicks straight down into the tubes and carries the resin with it. To build a film you need a certain thickness of finish sitting on top of the wood. On raw end grain that's close to impossible, because the finish keeps draining into the straws before it can pool.
So those first coats aren't finish. They're filler. Every coat you put on bare end grain is being swallowed to plug the open tubes, and only once enough of them are plugged can a surface film begin to build at all. "Just keep adding coats" isn't wrong, exactly. It's just the unwitting version of sealing. You're filling the tubes by accident, one expensive coat at a time, instead of on purpose in one pass.
The fix is to seal first. Lay in a thinned coat of whatever finish you're using, or a light wash coat of shellac, specifically to plug the tubes so the topcoats build a film instead of vanishing. A thin shellac cut is the thing here. It soaks in and seals without leaving a brittle slab on the surface. How much sealing depends on the wood. Ring-porous stuff like oak, with big open pores, takes more. Tighter, diffuse-porous wood takes less. Don't go hunting for a magic number of coats. Look at your wood and read the pores.
Two things will bite you if you don't know they're coming.
The first is raised grain, which is worse on end grain than anywhere else. On a face, water from a finish swells the surface fibers a little, you scuff them flat, and you've lost almost nothing. On end grain the water goes deep into those tubes and pushes fibers out from down inside, like sections of a telescope sliding open. When you sand that fuzz off, you're not just knocking down a whisker. You're cutting away the sealer you just laid and exposing fresh raw fiber underneath. So you have to seal again after sanding. Sand once and move on, and you've reopened the wood you thought you'd closed.
The second is checking, and this one is about timing more than technique. A cut end loses moisture many times faster than a face does. That uneven drying is what cracks it. If you seal the end soon after you cut it, you slow the moisture loss and keep the gradient from splitting the wood in the first place. But if you wait until the end has already checked and then put a film over it, you've sealed the moisture problem inside. The gradient can't equalize, and you get cracking that keeps working its way up through the finish for as long as the piece is in use. Seal early to prevent checks. Don't film over checks you can already see.
None of this is exotic. It's one extra step in the schedule, done before the coats that show. Seal the end grain, build on the seal, re-seal anything you sand back to raw. Do that and the end matches the face on the same number of finish coats as the face, instead of swallowing three to the face's one.
The next time you watch a coat disappear into the end of a board, don't reach for the can again. The wood isn't asking for more finish. It's telling you it was never sealed.