Finishing: When to Varnish or Clearcoat
Somewhere along the way, I've found that fully finishing a craft project is an incredibly important step. It's better for aesthetic reasons—the texture of paint beneath your hands feels oddly amateur on most projects. It's also MUCH better for durability. Cheap acrylics and even more expensive hardware store paints really aren't meant to hold up to the chips that are inevitable during handling and the daily beat of the sun.
Here's a quick overview of what finishing options are out there, mostly for craft projects involving paint and wood.
Varnish is the traditional finisher for just about anything wood. Use varnish to finish wood furniture, of course, but you can also use it on nearly anything wood, painted (such as murals), paper, fabric and clay like sculpey.
A clearcoat is basically a varnish. Neither the words varnish nor clearcoat refer to the chemical makeup of the coat, though, and since varnish traditionally refers to wood-based projects, the finish over metal or plastic is a clear coat. This can be useful when you're trying to figure out the best product for your project, but don't get too hung up on it.
Urethane and polyurethane do refer to the chemical composition of the substance. In general, polyurethane is the most easy to find, longest lasting and convenient type of clear coat finish. In particular, if you expect your project to get cleaned with mild chemicals (like, it's a table or kitchen trivet) you definitely prefer polyurethane. It's a pretty general term, though. There's still a wide variety of polyurethanes.
Acrylic, enamel and lacquer are all types of paints that can come in a clear finishing coat variety. I wouldn't bother with acrylic, enamel, or lacquer finishes on a general use craft project when polyurethane is an option, but there are some scenarios where they make sense. In general, use the same rules you use when comparing acrylic versus enamel versus lacquer.
Acrylic dries naturally more matte or at least not quite glossy (I believe there are glossy acrylic clear coats, but be careful an acrylic gloss isn't actually an enamel sold to go over acrylic paint). Also, acrylic lends itself well to spraying, and if you're using a spray on finish there's a good chance it's acrylic. While acrylic is waterproof, it isn't great for being regularly exposed to water (like, on a table that you clean after meals).
Enamel dries naturally glossy, is a bit more brittle and more likely to flake after dings when you have multiple coats, but wears to rubbing slower. It's considered more a home improvement product than a craft product. I believe it was more the standard before polyurethane came along.
Lacquer is known for its extremely smooth, glossy finishes, but it takes quite a bit of effort to achieve that perfection. Also, like enamel, that smooth finish is also brittle and can lead to problems with chips. For example, if a small chip happens in acrylic, it happens, but in lacquer you have more worry about the chip spreading and more of the coat flaking off.
Spray, brush or foam
The application of a varnish or clear coat can be almost as important as the substance used. In general, a spray is best for a very uneven surface (like canvas, fabric, or a painting with visible brush strokes). A spray on a completely smooth surface is much harder to get even and smooth itself, so I would avoid it on furniture, etc. Brushing varnish is probably the most widely useful method, but when possible I'd recommend upgrading to using a disposable foam “brush” and thin the varnish slightly so it's more of a liquid than a gel. This might take a few more coats, but, cuts way back on problems with air bubbles and ridges and uneven drying.