I see a lot of discussion over various resins now so I thought I'd chime in again.
First, Tung oil and linseed oil are similar, but it's very important to note that when we are suggesting using these finishes, we are always suggesting using POLYMERIZED Tung oil, and BOILED Linseed oil. Even if we don't state it each time, it has to be polymerized / boiled. Do NOT use 100% pure, raw Tung oil, or you're looking at a drying time on the order of 45 days, same with raw linseed oil. And yes, as others have said, the difference between them is that Tung oil does not yellow as much over time.
Second, please be aware that there's almost no brand of Tung oil out there that is actually selling Tung oil. They're almost ALWAYS thinned, and mixed with other resins. The Watco Tung oil Finish is actually a Tung-oil and alkyd resin blend, per its own TDS. Its NOT pure Tung oil. It's closer to an old-school varnish than a true Tung oil finish. They no longer make their pure Tung oil product. The ONLY 100% pure, polymerized Tung oil I've been able to find at a big retailer is Lee Valley's Polymerized Tung Oil.
Danish oil is a blend of Tung oil, linseed oil, and a variety of resins, like Rosin (derived from pine trees). It forms a much thicker film than tung or linseed oil, and is more like a "varnish" of old.
Then you have actual "varnishes", which I put in quotations because there's no formal definition of varnish, but essentially its any resin, dissolved in a solvent. Typically, though, we use varnish to denote products whose resins are naturally-derived, as opposed to synthetic, but this is not a guarantee.
Then there's shellac, aka dissolved bug poop flakes. It's an extremely brittle and delicate finish, not suitable for real protection, but it has the ability to bond to damn near anything, and is often used as an intercoat for that reason. It also is used to produce a French polish finish.
To cover something in shellac, which is made of Lac, was called "lacquering".
In modern parlance, athough, lacquer is only actually lacquer if it's based around nitrocellulose. It dries super hard, super fast, and is super shiny. There are many products being advertised as lacquer which are not, and are simply being advertised that way to promote how they also dry fast and shiny and hard. They're usually acrylic clearcoats.
Then there's acrylic products, usually just referred to as acrylic enamels. They are based primarily around acrylic resin.
Then there's urethanes. Spar Urethane for the outdoors, poly urethane for indoors. It's based around Cabamate as its resin, and there's no coating out there that's tougher, or more abrasion resistant than Urethane. Many people find they look plasticky, but those are only the cheap ones, or if they're applied incorrectly. A high-quality poly is crystal clear, and can even be used to do a pseudo French polish.
