How do I get smoother surfaces on small keychain prints?

Smaller layers will help on those curved edges, but less than you'd hope on flat tops. The ridges you feel with your thumb are mostly on the curves because the nozzle keeps stepping sideways and up, so dropping from 0.2 to 0.12 makes a real visible difference there. On flat-ish surfaces the layer height barely matters, it's more about flow and temperature. If the filament's running a bit hot the edges bulge slightly and every line shows. Try knocking the temp down 5 or 10 degrees on a test keychain and see if the surfaces calm down before you commit to longer prints.

The gray thing is normal, don't blame yourself there. Dark colors like black absorb light and hide the shadows between layers. Light grays and whites are the worst for showing lines because every ridge catches a tiny highlight. If you want smooth-looking gifts without the print time, a silk or glossier gray will fake it better than a matte one ever will.

Two ways to dodge the doubled print time. One, check if your slicer has an option to use variable layer height, finer only on the curved outline and 0.2 everywhere else. That gets you most of the visual win for maybe 20 to 30 percent more time. Two, many slicers have a surface smoothing pass where the nozzle sweeps the flat top with no extrusion, basicly melting it glossy. Costs a couple minutes per print, does nothing for the sides, but makes the faces look great. For sanding coin-sized stuff, regular sandpaper is misery. A fine grit nail buffer block from the drugstore works way better, you can hold the keychain and buff the flat sides in seconds. For the curved edges wrap some fine grit around a toothpick. Honestly though, for 50 of these I'd fix it at the slicer and skip sanding entirely, you'll rub the logo edges soft before they're truly smooth at that size.

One thing if you do try any sanding: bump the walls to at least 3 perimeters or you'll burn through to the infill and it looks way worse than lines. I learned that the hard way on a batch of 80 badge charms for a school fair a while back, sanded two of them into little donuts before I figured out what was going on. With enough wall thickness a quick buff on the flats takes 30 seconds each and the outlines stay crisp. Dry your filament too, gray especially picks up moisture fast and damp filament makes those ridges pop even more.

I ran into the same thing making little tags for my workshop drawers last month. What saved me was flipping the model so the flat logo face printed directly on the bed. That side comes out almost glassy, and the ridged edges ended up on the back where nobody looks. Might not work if your logo is recessed rather than raised though, I'm still figuring out when that trick applies. Does your keychain design sit flat on the plate easily or is it one of those shapes that needs supports no matter what you do?

Before you print all 50, do one test in the gray and one in the black and put both on a keyring in your pocket for a day. Gray wears its scars proud, always has. Way back we used to hide this stuff with a light coat of spray primer meant for models, fills the valleys without eating detail, then a dab of color on top if you care. Slower than it sounds but you can do a whole tray at once.

If that gray is ABS or ASA you've got another old trick available too, though test one first before you commit the batch.

For coin-sized stuff I gave up on sanding years ago. I batch out beer opener tags for my brother's shop and a quick pass with a heat gun on low, moving constantly, melts the outer surface just enough that the lines soften up. Ruined the first two by lingering too long, learned to keep it waving like I was drying nail polish. Takes about ten seconds a piece once you get the feel for it. Worth testing on your worst looking one first though, gray filament tends to go glossy unevenly if the temp's already a touch high.

Update: the flip suggestion worked a treat. Printed a test batch logo-face down and that side came out smooth as anything, way better than fighting the ridges. Downside is the curved edges on the sides still look scratchy in the gray. That primer trick someone mentioned, does it work okay on PLA? Never used it on prints before and don't want to melt the detail off.