The last coat isn't about building the finish. That's done. The last coat is about sealing what you built and not screwing it up on the way out.
Every coat before this one was adding something. Film thickness, pore fill, the surface coming together. This coat adds almost nothing. It closes the door. And any mistake that makes it through lives with the piece forever, because there's no sand-and-recoat coming after this one.
The Last Sand
You've been sanding at 320 between coats. Before the final coat, go finer. 400-grit, or the grey Scotch-Brite pad. You're not leveling anything. The surface is already built. You're just smoothing the finest scratch pattern from 320 so the last coat lays down over glass.
After this sand, run raking light across every surface. This is your last chance to find a dust nib you've been sanding over for three coats, a hollow where the film ran thin, an open pore on red oak that you thought was filled. After the final coat cures, finding any of these means stripping back to wherever the problem started. That's a day of work, minimum.
Tack cloth. Then tack cloth again. If you're serious about this, wet the floor around your work area and don't stir up anything in the shop for an hour before you start. Dust in the air doesn't care about your schedule.
Putting It On
Same technique as every previous coat. The difference is pace.
Waterborne dries fast. On the final coat, that speed becomes the enemy. Lay it on in even passes and walk away. Don't tip off a second time. Over-brushing marks on the final coat are permanent, because there's no 320-grit session coming to save you.
Oil-based varnish, you can go slightly thinner than the earlier coats. You're not building film thickness anymore, so a thinner application levels better and produces a finer result. Flat surfaces: foam applicator. Shaped surfaces: brush.
Hardwax oil follows the same discipline as every coat before it. Apply, wait for cloudiness, wipe off every trace. The surface shouldn't look wet when you're done. It should look like dry, protected wood with depth behind it.
Then leave the room. Don't sand nearby. Don't sweep. Don't breathe on it if you can help it. Every particle in the air is looking for a wet surface to land on.
Dry Is Not Done
This is where more finishes fail in service than in application.
Waterborne is dry to the touch in an hour. Handle-hard in two to four hours. Fully cured in two weeks. Everything before full cure is provisional. Your dining table "feels fine" at day three, but it's not ready for a hot mug or a sweating glass. Not yet.
Oil-based finishes cure by cross-linking, not just evaporation. The surface feels solid at twenty-four hours. Full cure is thirty days. Put a hot mug on it at seventy-two hours and you'll get a white ring that won't buff out. At thirty days, you won't.
Hardwax oil is the fastest to cure. Twenty-four to seventy-two hours, typically. That's one of its real advantages for pieces that need to go into service quickly.
A good rule: don't put the piece into use until it's past 70% of cure time. For dining tables, wait the full period. The finish earned that time. So did you.
Adjusting Sheen After Cure
If the sheen came out a little hotter than you wanted, the fix is 0000 steel wool over the fully cured surface, dry, with light even pressure. This cuts the microscopic peaks in the film and brings the sheen down toward satin without touching the quality underneath. Follow with paste wax, buffed out. The wax fills the steel wool cuts and produces a hand-rubbed surface that feels better than any finish straight from the can.
One thing. Don't use steel wool on waterborne finishes. Steel wool leaves ferrous particles behind and they will rust under the film within days. You'll see small brown specks appearing across the whole surface, and there's no fixing it short of stripping. Use the synthetic 3M pad (grey or white) for waterborne. Same technique, no rust.
Paste wax over a cured oil-based or hardwax oil surface is also maintenance, not just a sheen fix. Once a year on a working surface extends the finish life significantly.
What Done Looks Like
Run your hand across the piece. The surface is continuous. No rough patches, no skips, no spots where you can feel bare wood through thin film. The wood shows through the finish, not under it. The sheen is even from every angle.
Under raking light: no runs, no sags, no fish-eye, no dust nibs casting shadows. The surface doesn't call attention to itself. You're looking at the wood.
Pick the piece up if you can. The weight hasn't changed. But something has. It feels finished. It feels like a thing that's complete.
That's what done looks like.