Is it normal to have small gaps where the mitered return hits the casing?

The uneven subfloor is almost certainly why you're seeing those gaps. When the baseboard doesn't sit perfectly level, it throws the vertical angle of your miter off just enugh to leave a sliver at the top or bottom. Try keeping a few **shims** handy to level out the base before you nail it. If the floor is relly wonky, you might need to scribe the bottom of the baseboard so it sits flat while staying plumb. If the base is tilted even a degree or two toward or away from the wall, your 45-degree miter isn't a true 45 anymore relative to the casing. For those tiny return pieces, stop relying on just nails. Use a high-quality **CA glue** with an activator. Glue the return to the main piece of baseboard while you’re still at your saw station, then install it as one solid unit. It’s way easier to get a tight joint on a workbench than it is trying to pin a half-inch piece of wood against a wall. It also prevents the wood from splitting, wich happens constantly when you try to shoot an 18-gauge brad into a tiny return.

Since your casing is thinner than the base, you can also try a slight **back-bevel** on the return piece. Cutting it at 45.5 or 46 degrees instead of a dead 45 can help the outside edge of the miter close up tighter. While paint-grade trim is meant to be finished with wood filler and caulk, you want the wood-to-wood contact to be as tight as posible. If you can see daylight trough it, it's usally a fitment issue, but a hairline gap is somthing a painter can easily handle. Don't beat yourself up over the time spent. Five months in, you shold be focusing on the mechanics of the cut rather than the clock. Speed comes once the muscle memory for setting the saw and handling the glue kicks in. Just make sure you aren't over-nailing near the ends, as that can sometimes pull the joint apart if the wall isn't perfectly flat. If the wall is bowed, sometimes you have to shim behind the base to keep the miter from opening up when you nail it off.

You shold check for an accumulation of drywall compound at the intersection of the wall and the subfloor, as this common structural imperfection frequently induces a slight outward flare at the bottom of the baseboard that renders a standard vertical miter geometrically impossible to close perfectly without first excavating that excess material. While the previous guy correctly identified the subfloor as a culprit, the hidden taper caused by sloppy mudding is often equally insidious, necessitating either a sacrificial back-cut on the rear of the trim or a thorough scraping of the wall base to ensure the material registers flat against the vertical plane of the studs.

I’m still just learning the basics, so I honestly didn’t even know mitered returns were a thing until recently. I usually just butt the wood together and hope for the best, which probably explains why mine look so rough.

That CA glue tip sounds like a game changer, but I have a dumb question. When you're cutting those tiny return pieces, how do you keep them from getting sucked down into the saw? Every time I try a small cut on the miter saw, the piece just disappears into the hole or shatters. Do I need a special fence or something to keep that from happening?

Quick update — I tried scraping out the drywall mud buildup like you suggested and it helped the flare alot. I also gave the CA glue trick a shot for the returns. Since the subfloor is so wavy, do you guys think it’s better to scribe the bottom of the baseboard, or shoud I just keep using shims to level it out?