Thoughts on Finishing and Finishes
What follows is my experience derived expert opinion.
All finishes are based on either drying oils or resins (or some combination of the two). Drying oils such as Boiled Linseed Oil and Tung Oil, which are commonly used in wood finishes, dry (cure) through a polymerization reaction in the presence of oxygen. BLO and Tung Oil are usually heat treated to speed up the chemical reaction, but can also be used raw. Resins, which are the bases for film finishes can be either polymerizing or hardening by solvent release.
A few words about so-called Sealers. There’s no such thing because there’s nothing to “seal.” Any finish, be it oil or resin based, needs nothing under it to perform its intended use. The argument that a “sealer” allows the top coat to adhere is specious. If the final finish won’t stick to the wood, then it’s the wrong finish to begin with. The argument that a “sealer”, shellac in particular, is needed to prevent a stain from bleeding into or reacting with a top coat only means that the stain and top coat should be compatible, thereby voiding any need for an intermediate barrier. And as far as a “sealer” filling the open grain before the top coat film finish, the top coat will fill the grain just as effectively without the added complication of a different formula. Simply consider the first (thinned) coat of the final finish to be the “SEALER.”
On to the individual finishes:
• Linseed Oil – Raw linseed oil is useless to the woodworker, and used outside it’ll turn black from mold. Boiled linseed oil is next to useless to the woodworker. BLO is slow to cure and offers no protection from anything, regardless of the number of applications. The prevailing myth is that it makes the grain “pop.” Rather, its dirty amber cast highlights contrasts in the grain by making some areas muddier than others while making any wood dull and lifeless. The best use for BLO is as an accelerant for fire bugs.
• Tung Oil – Raw tung oil is the consistency and color of warm pale honey that can be applied directly to bare wood, highlighting the grain and imparting a clear warm glow. Application is wipe-on/wipe-off, which can be made easier by mixing it with an evaporative solvent like mineral spirits or naptha. Multiple applications will produce a close-to-the-wood finish with pretty good resistance to many household liquids. The drawback to using raw tung oil is the long time it needs to cure, days and days rather than hours. Tung oil that has been heat processed and combined with solvent vehicles and driers does cure within hours so that multiple coats can be wiped or brushed on in a shorter time. Once the initial coat is well into polymerization, subsequent coats will cure faster. The most popular brand is Waterlox, which expensive at around $30 a quart and once opened it will want to jell whatever’s left in the can. However, Waterlox does deliver on its claims and returns a handsome finish. Caveat emptor: Many products include Tung Oil in their names while they have none or only homeopathic amounts in their formulas; there’s no Truth In Labeling Law when it comes to paints.
• Shellac – It’s been around for a long time as a finish, either brushed/sprayed on or as a high gloss French polish. Forget about French polish; it’s not worth the steep learning curve and fussy process when better alternatives are out there. Brushing it on ain’t no cakewalk either. It tends to streak and brushwork needs to be fast and sure to prevent earlier coats from softening and being pulled. Also, it has poor resistance to water and household chemicals. Finally, if you want your furniture to be your legacy, shellac will eventually dry out, crack, and flake away. Sealer? Bah!
• Oil Varnish – The predominant contemporary varnish is Polyurethane, a manmade resin based, tough and durable film finish that’s highly resistant to water and household chemicals. Its amber tone imparts a strong warming cast to the wood. It’s easy to apply by brush or wiping and cures enough over several hours to reapply, which can be a drawback, as a fully polymerized coat will inhibit adhesion of a later coat. Recoating intervals should be kept to 24 hours or less to avoid that potential problem. Poly is available in a range of sheens and, once fully cured, can be rubbed out with wet sanding, steel wooling, and polishing compounds. It’s a finish that one can learn to love for its friendly application requirements, physical properties, and appearance. Those who demean it haven’t learned to use it.
• Waterborne Polys and Acrylics – My experience is limited to the brands usually stocked at retail stores, my favorite being the Varathane line. Available in several sheens, it goes on easily with brush and can be built up with successive coats in a couple days. I haven’t tried it, but I’ve seen work that’s been sprayed that is indistinguishable from other fine finishes. Soap and water cleanup. It cures to a durable resistant finish, and in the version made for floors that contains aluminum oxide, it’s as tough as any other finish – great for tables that are gonna be danced on. It can be rubbed out just like oil polys. Some folks see a bluish cast in it, but rose colored glasses will fix that.
• One-Part Nitrocellulose, Acrylic, and Alkyd Solvent Lacquers – The best of all, IMO. With a decent compressor and gun and a little practice, spraying lacquer is the most consistently successful means to a great finish. From bare or stained wood to final coat, application can completed in one several hour session, including cleanup. It’ll go on over dye stains or cured oil stains, and can be naturally toned as in the case of NC or tinted with dyes and other colorants like metallics and pearls to deliver spectacular results. The acrylic and alkyd formulations are water white, allowing the wood’s natural beauty to be revealed. Available in various sheens straight from the can, they can also be rubbed out and polished. They’re tough enough for kitchen and bath use, and a ding in the finish years after the original can be erased with a fresh application that melts into the old. Sherwin-Williams is my preferred brand.
• All Other Finishes – No experience; no opinion
That’’s my story and I’m sticking to it,
-- Clint Searl....Ya can no more do what ya don't know how than ya can git back from where ya ain't been