It’s been interesting to be working with various stains and finishes lately. I prefer to have specialists do finishing, but as a woodworker, we wind up doing our fair share.
Stains are (mostly) pigments or dyes.
Pigments are, essentially, solid colors ground to a very small particle and suspended in a solvent of some sort. They will soak into wood some, but basically sit on top of the surface. With open grain woods, like Oak, pigments will fill in the open grain and can enhance the grain pattern. On closed grain woods pigment stains will often wipe off, especially if the wood is sanded too smooth. This is why pigment stains contain binders- basically glue to hold the stain onto the wood.
Pigmented stains are a lot like paint, and there are some “stains” that are designed to act like paint. Arborcoat is a solid pigmented “stain” that is painted on, but not wiped off later. The binder is designed to be strong enough to hold a full film coating onto the surface.
Dyes are soluble salts that, when dissolved, are much smaller than a pigment. With closed grain woods, like Maple, a dye will react to the different grains in the wood and can enhance differences in the grain. With open grain wood the effect can by to muddy the wood as it soaks into the open pores and dyes the rest to a similar color. Dyes can be used to add much more, and more vibrant colors than pigment stains- that flamed Maple Fender Stratocaster most likely has a dye.
In general, stains are designed to color or tone wood, possibly to bring out or highlight grain patterns. Stains are then coated with a clear finish to protect the wood, and perhaps the stain, also.
Penetrating and Film Finishes
There are basically two types of finishes that are applied to wood (and stone, for that matter); Penetrating finishes and Film finishes.
Penetrating finishes are generally oils- Tung oil, Linseed oil, Flaxseed oil. The oils used to finish wood need to be stable, won’t turn rancid (start to smell), and this generally means they dry to a hard finish.
Tung oil and Linseed oil are commonly used to finish furniture, taking many, many coats with extended drying times between each coat (even with drying agents, these oils often require 12 hours between each coat). As each coat is applied (often wet-sanded or buffed between coats) it becomes glossier and harder.
Many deck coatings, such as Cabott, are penetrating oils, often with a pigment or dye in them to create a more uniform look to the deck.
Penetrating finishes cause the wood to become more moisture resistant by filling in the outer layer of the wood, causing it to become more water resistant.
Film finishes sit on top of the surface, binding to it through chemical and mechanical means. Examples of film finishes are Paints, Lacquers, Varnishes. Film coatings protect the wood by coating it in a hard surface. The benefits to film coatings are that they are continuous over the surface and do a better job of preventing water and sunlight to damage the underlying surface.
Film coatings are good in climate controlled places and on non-wearing surfaces. IE, paint is better than a penetrating finish for most walls because it doesn’t get a lot of abrasive wear. Decks generally are not painted because they quickly start to peel from wear; as the paint is cracked or scratched, water can get underneath the film and breaks the bond. Floors, which generally get a film finish these days used to only be oiled with a penetrating finish. Today we can use film finishes, but they are expensive and require extensive preparation and careful application.
Lacquers and Varnishes
There is a historical difference between these products, but as time goes by and technology advances how we can create and suspend polymers the distinction becomes more of use than of ingredients. There are water based “Varnish” products, Epoxy “Lacquer” coatings. Even the concept of a water based Lacquer requires that the ingredients be something that hardly resembles the original. So the following is a light explanation of my reading on the subjects and is based partially on my own bias and not-deep understanding.
Varnish was originally tree resins dissolved in turpentine. As the tree resin dries it hardens into a protective film.
Today most Varnishes are Alkyd resins that use mineral spirits as a solvent and linseed oil as the carrier. The benefit to Varnish originally is that it dried slowly to a very hard, thick, coating. That has been maintained as a primary difference in how Varnish is used- high solids (solids can be thought of as how much wear the product can take).
Lacquer originally referred to a resin secreted by the Lac bug. It was used extensively through Asia. Shellac is the pure resin, cleaned, and is soluble in alcohol, which makes it very easy to use. It also can be a food-grade finish, depending on how it is processed; it is sometimes used to coat apples and some candies to give them a shiny look and to prevent them from melting. Shellac dissolves in alcohol.
An interesting thing about Shellac- it has the reputation of getting “rings” from water glasses, etc., is actually the wax absorbing the water. Wax is used to protect the Shellac from scratching or dissolving should someone spill their wine or whiskey. Shellac is very brittle, scratches easily, and dissolves in alcohol- French polish’s benefits; oil to make the wood grain pop and help keep the shellac a little more plastic and wax to protect the shellac from alcohol. Shellac naturally has some wax in it, and that can cause difficulty with some finishes, and the wax gives Shellac an orange-tinge. Fortunately, it is easy to de-wax Shellac, and de-waxed Shellac is a (nearly) universal covering. It will stick to nearly anything, and just about everything sticks to it.
Today lacquer generally refers to a polymer dissolved in volatile organic compounds. The early polymer resins were Nitrocellulose, and that is still available, but is being displaced by Alkyd and Acrylic.
Okay, now what’s an Alkyd? And what is an Acrylic?
Alkyd is a Polyester made of alcohol and organic acids. Alkyds are the most common resin in “oil based” coatings. Alkyds are flexible, take color well, and last relatively well.
Acrylic is a plastic created from an acrylic acid. Lacquers mostly use an emulsified Polymethyl Acrylate. Wikipedia explains that “Acrylic resin is a general term for any one of the plastics (resin) generated through chemical reaction by applying polymerization ninitiator and heat to a monomer.”
Now that we have an idea of the fancy films, that information translates well to paint.
Latex is a rubber derived from many plants and trees. The famous “rubber trees” were first used to make tires, but other more reliable methods of creating latex quickly supplanted that. While we can buy latex gloves that have natural latex in them (which is what some people have allergic reactions to), virtually all latex paint is made of synthetic latex or vinyl.
Acrylic latex is Latex paint (that may be vinyl) with Acrylic added to create a stronger coating that wears longer and retains color better.
Solid Acrylic paints are more color stable and have better wear than Latex or Acrylic Latex paints, but the formula.
Enamel. Enamel used to be synonymous with a tinted Varnish in that it was an oil based coating that dried to a thick, hard surface. Today it is mostly a term meaning that it dries to a very hard finish. Oil based enamels are mostly Alkyd resins, while water based are Acrylic or resins. There are also Polyurethane enamels.
Polyurethane. Polyurethane coatings are a stronger plastic than Acrylic. Most automotive paints are Urethane paints, and the chemistry gets very complicated. Polyurethane paints comes in three types- Water-based, Oil-based, and Oil-modified Water-based! Like Acrylic, Polyurethane can be added to Latex paints to increase the durability, although there are some color concerns. Oil based Polyurethanes tend to yellow.
Well, that’s enough for now. More later!