Reddit mentions of Swisspers Premium Cotton Rounds, Hypoallergenic, 80 Count, Pack of 6 (480 Rounds Total)

Good guide. Especially the method of drying. ;-)

That said, I recommend against nail polish and nail polish remover; polish doesn't set as well as enamel paint, and thus, it won't last as long or look as nice. One factor to this is that enamel paint settles flat better than nail polish does - the latter tends to curve inward or bulge outward, depending on exactly how much there is, whereas enamel paint can be polished off relatively even to the gun surface. Here's a comparison, with nail polish on the left and enamel paint on the right, and here's the final product. Note that this method also works on plastic; I filled the etching of the Crimson Trace laser as well.

Instead, I recommend Testor's 1168 Flat White enamel paint, which can be found at most hobby supply and arts & crafts stores. I apply with a very thin brush or with toothpicks, depending on how fine the work needs to be. I'll stick the tip directly into the etching grooves so that I don't end up with bubbles. I try to keep the coat relatively thin, but just thick enough so that I can't quite see the etching through the first coat when I'm done. (More on the second coat shortly.) Let it dry for an hour with a fan on it. You can sometimes get away with less time, or use a hair dryer to speed up the process, but I've found less than an hour (if you don't use a hair dryer) leaves the paint too tacky and more comes off than I'd like.

To clean up the excess paint, don't use paint thinner - not even Testors. Paint thinner will, well, make the paint too thin. This makes it too easy to accidentally remove paint from the etching that you want to leave behind. Instead, I use Hoppe's No. 9 Bore Solvent. It works similarly to paint thinner but isn't quite as powerful, which means it doesn't thin the paint as much. It's also a useful gun cleaner, so it's handy to have around anyway. :-)

Use a combination of cotton rounds and makeup sponge wedges, both of which can be found in the make-up section in most grocery stores or pharmacies for a few bucks per bag. The rounds are good for broad strokes to wipe away larger amounts of excess paint, and the wedges are good for detail work or to gently swipe evenly over the etching without pressing loose cotton threads into the etching. I try to do as few wipes over the actual etching as possible - the more you wipe the etching, the greater the chance you remove paint from inside the etching. Don't press hard, either; just press enough to maintain contact; let the solvent do the work for you.

Let the paint dry for another hour, and then apply a second coat. Odds are good that you've accidentally removed too much paint in some parts of the etching, especially where it's thin or has sharp corners. Then let dry for an hour, remove the excess with Hoppe's again, and let dry overnight to fully cure.

This will probably take you around a total of 3 hours to do, but it's worth the extra time - it looks beautiful and will last forever. Obviously, you'll need more beer for this method, but OP says you can get a 30-pack of Miller Lite for $18. It may suck, but after 30 of them, you probably won't care.