Relative Merits of Finishes

Finishing is a real art, and each of the types of finish has relative merits. Some are harder to apply, some are easier. At some point I'll walk through my approaches for what they are worth. For the beginner though, some guidance can be had here I think.

Your basic finish types are reactive vs. evaporative. Reactive finishes form one-way chemical bonds that will not break when their solvent is re-applied. Evaporative finishes will re-dissolve in their solvent. There is a third type, Coalescing finishes, which are typically waterborne and feature some evaporating and some reacting.

The main reactive finishes one will encounter as a beginner poking around through Home Depot or someplace similar are: Tung Oil, Linseed Oil, Danish Oil, Polyurethane and other Varnishes. Note that Waterborne Polyurethane or "Crystal Clear" Polyurethane is a Coalescing finish, covered later.

Tung Oil and Linseed Oil need only to be applied sparingly, unless being wet sanded for pore filling. Their purpose is mainly to impart color and depth to the wood. Linseed Oil on its own may never dry, so the version people should buy is "Boiled" Linseed Oil (this contains catalysts that speed the drying up to about 24 hours). Both Tung and Linseed react with oxygen in the air and change to a solid... this is not drying like an evaporative finish, but it is a one-way reaction. They are easy to apply: dip a rag, and wipe on sparingly. No errors, no problems. Not very durable, and little resistance to anything. They can impart a darker, more dramatic color and figure than a varnish will alone, and varnishes can be applied over oil.

Polyurethane and Varnishes also chemically react with the air and are a mix of one of the above oils, and a resin. Drying takes a couple of hours (the solvent boils off), curing for full hardness can take a week (bonds between the molecules form). They are difficult to work in general, the manufacturers have to ship them minimally thinned so that only a pro could apply it due to EPA regulations. Even if you thin them out, you get problems like runs, sags, dust, brush marks, dried varnish flakes, pools... you name it. These can be dealt with, it's a subject for a new posting though. Overall, difficult to apply, but very durable and resistant to spills. They are difficult to repair. Polyurethane is technically a Varnish but is usually spoken about on its own. Polyurethane in particular breaks down in UV light, so if using outdoors, you absolutely must buy an outdoors version which will have UV blockers in it.

Danish Oil is basically a long oil varnish. It has very little resin in, so applies easily like an oil, and features more resistance than an oil finish would have due to the resin. It's a great choice for the beginner provided the piece is not going to get a lot of wear. You can get very beautiful results. I like the Deft danish oil product.

The Evaporative finishes that one will usually see are Lacquer (Nitrocellulose) and Shellac (a resin secreted by the Lac beetle). Because these finishes can re-dissolve in their solvents, they can be repaired easily. Both of these finishes impart very dramatic effects and are very clear. They both dry very quickly, Shellac can dry in seconds. Overall they work in a way that offers much fewer surprises and "wtf" moments than Poly and Varnish offer. Shellac is more difficult to work with because of its rapid dry time, but otherwise the two are very similar.

Lacquer ships dissolved in a real witches brew of solvents... if these solvents come in contact with any of the other finishes usually a big mess is the result. Of them all, this is the one that you do NOT want to get on your skin. Lacquer is more durable than Shellac, but both can be hard to apply and get good results. Best results are usually considered to be had by spraying. Watco though makes a brushing Lacquer that I am very happy with, for me it flows out much nicer than Deft's product, and it leaves little to no brushmarks and bubbles are a non-issue. Lacquer is generally considered the best all-around finish when weighing out the pros and cons. Because of the witches brew of solvents, Lacquer dries to touch quickly but has a time where it is almost the consistency of a gel for an hour or two... after this, depending on how thick and how rapidly you put it on, it make take a week or more for all of the solvent to completely evaporate off. This for Lacquer is the cure time, where final hardness is achieved.

Shellac on the other hand is only dissolved in alcohol. This can really dry your hands out if you stay in prolonged contact (and be painful), but if you spill some on yourself it is no big deal. Best results for the amateur with Shellac are usually from padding. For this, you dilute some Shellac using a squeeze bottle and alcohol, dampen a pad with it, and rub on quickly. It will dry seconds after your pad is gone, so there is a need for a light and quick touch here. When properly applied, Shellac is probably the best looking of all the finishes. To apply it properly though, is an art for in and of itself. Shellac will also cure under the same principles as Lacquer.

The Coalescing finishes are those that are waterborne. They will say somewhere on the can that they should be cleaned up with soap and water (which is a nice feature), they have much less solvent (another nice feature), and many of them are completely colorless, which may or may not be a nice feature depending on what you want. These finishes work by suspending particles of solvent bonded to resin in a bath of water. When the water evaporates, the solvent activates and the resin particles become bonded together. The bonds are not as strong in general as those formed by polyurethane, but this technology is constantly improving and will probably be the way of the future.

The main downsides of these finishes are that they are extremely difficult to apply in any way other than spraying, they don't level very well and they can easily foam and bubbles can get locked in because they dry so fast. Furthermore, most of them look cold, and cheap compared to any of the above finishes. Manufacturers are desperate to advance these technologies and convert users of traditional finishes, so (in my opinion) make a lot of Big Claims that do not bear out in practice. The hardnesses of these finishes are exaggerated ("diamond hard"), the ease of application is exaggerated ("formulated specifically for brushing and padding" says one manufacturer... except, not.).

The one thing that generally holds true is that they do clean up very easily and tools are easily reused. Sometimes the uneven leveling common to these finishes will clear up overnight as slower reactions in the solvent take over from the initial evaporation of the water. I've found that the best waterborne finish out there is the common Varathane. All of the higher end "premium" waterborne finishes that I've tried have not met up to the claims that the manufacturers have made, and none of them settled out as flat as Varathane has (in some cases, perfectly flat). So, sprayed on Varathane from the can is actually a decent choice for a beginner or DIY-er. Note that the "Professional" Varathane is an oil-based finish, not the waterborne I'm speaking about here. In general these will be more durable than Shellac and Lacquer, but less durable than Varnish.

So for the beginner, the best two finishes to use would be Deft's Danish Oil, and a spray can of Varathane (not on the same project!). Both of those are very hard to screw up if you follow the directions and both will give very different looks and durability. From here, move to Lacquer at medium skill, and then to Varnish or Shellac at high levels of skill.

The only finishes you should not worry about a solvent mask are Shellac, and Tung or Linseed Oil. Everything else has a bad solvent, even waterborne, don't breathe it and apply in a well ventilated environment.

I'll write more one day on what I do to apply each of these. Finishing is usually a struggle, but with some perseverance, good results can be had.