What is a “Finish”?
Wood finishing is probably the most complicated step in any wood project, mostly due to the sheer volume of options and choices available. A basic definition would be: “A protective layer formed by a permanent or semi-permanent solid”. The solid portion of a finish is what is left behind after everything dries or cures. There are a myriad other terms around finishes that can be descriptive, confusing, redundant or even misleading. A quick search for wood finishes will probably reveal terms like, poly, urethane, shellac, lacquer, varnish, wax, spar varnish, acid cure, moisture cure, UV-cure, linseed oil, tung oil, mineral oil, butcher block conditioner, etc. just to name a few. While there are many varieties, the overall objective is the same, and that is to leave some substance (the solids) behind to protect or alter the appearance of the wood.
Chemistry and Physics of Finishes
The manufacture and use of wood finishes is a blend of chemistry and physics. The basic protection provided by a wood finish is that it leaves something behind that protects a wood surface. Wood is a porous substrate, meaning that it is full of voids and holes. These voids can trap dirt and liquids that can not only ruin the appearance of the wood, but can also lead to other issues like rotting, mold or mildew. The goal of wood finishes is to fill or seal off those voids to protect it. From a physics perspective, this is basically like building a wall. Overall, a wall is a wall and it will tend to keep things separated regardless of what it is made out of, to a point. What that wall is made out of is the chemistry side of the coin. There are lots of building blocks available that have their advantages and disadvantages, similar to your standard construction building blocks. You can have walls made from paper, steel, wood, brick, stone, concrete, glass, etc. just like you can have coatings made from oils, acrylics, urethanes, alkyds, natural resins, synthetic resins and/or waxes. Even beyond that, you can combine different building blocks to create an endless blend of ingredients. Just look around your neighborhood and look at all the different ways that buildings and houses can be assembled. You can have brick houses, concrete and glass houses, steel and glass sky scrapers, log cabins, adobe houses, fiberglass trailers, adobe houses, etc. They all accomplish the same goal of providing shelter, but they use different building blocks to achieve that goal.
Types of Finishes
While there are hundreds of options, ultimately, there are really two types of finishes, surface film-forming and penetrating. Some examples of surface finishes would be oil modified urethane, polyurethane, polycrylic, conversion varnish, waterbased urethane, spar varnish, lacquer, shellac, most paints, and aluminum oxide to name a few. Some examples of penetrating finishes would be raw/polymerized tung oil, bodied/boiled linseed oil, teak oil, butcher block oil/conditioner, Danish oil, wood stains, waxes, hardwax oils, mineral oil, and really any other type product labeled as “oil” (not always “oil finish” as that may have some film-forming properties).
The best way to differentiate between surface and penetrating finishes is to see how they interact with a non-porous surface, like glass. Film-forming finishes will dry into hard (usually clear) coatings. Penetrating finishes will mostly just sit on the surface and eventually dry into soft films or pastes or not even really dry at all (in the case of mineral oil). Also, because penetrating finishes don’t dry well into hard, clear films, you’ll most likely always see a “wipe off excess” (or similar) step in the instructions to ensure you do not have excess product left on the surface.
The biggest difference between the two categories will usually be the final appearance. Again, film-forming coatings are designed to form one continuous layer over the surface. This generally will give the surface a more “finished look”. It will look like there is a layer on the surface of the wood and will look more polished. This doesn’t necessarily mean glossy as you can have matte or flat surface finishes, but the overall surface will look and feel coated. Penetrating oils will have a more “unfinished look” as they are mostly below the surface and at best they create a very thin “film” on the surface. The surface will usually be lower in gloss, although a nice luster can be achieved. It will also look and feel a little more natural.
There are of course, always exceptions to the rule. There are some film-forming coatings that can be treated like penetrating finishes. These are usually almost identical to other film-forming finishes on the market, however they are very low in solids to allow them to soak into the wood better. If many coats of these are applied, they will eventually form a film, so they aren’t true penetrating finishes. Some examples of these would be very diluted urethanes or thin cuts of shellac to act as pre-stain wood conditioners or sealers.
Mixing and Matching
It is not uncommon to use multiple products on any particular project. There are a wide variety of products that are used for wood projects that all have their own unique properties, so many users look to combine and mix and match different products to achieve a specific outcome. Here we will look at some of the more widely used products and how they compare and match up with Waterlox products.
General Guidelines
There are a few considerations that you should take into account when considering your final layering system for your project. In general, penetrating finishes should not be applied over top of any existing surface film finish. This is because once the surface is coated/sealed with the surface finish, there is nowhere for the penetrating finish to go. This is similar to the glass experiment mentioned earlier. Conversely, putting a surface finish over a penetrating finish should not cause issues, unless there is an incompatibility in the chemistries of the different products. In fact, this case is common in applying urethane over a penetrating stain for color.
Another special case is when a surface finish is thinned or modified in such a way that it is designed to act more like a penetrating finish. This can be dilution of paint for white washing or thinning of a urethane or shellac to use as a “sealer coat” or as a wood conditioner. Overall, you are not going to change how that product will dry on glass, but the lower solids allow it to soak further into the wood surface. True penetrating finishes should still not be used over these products as a final coat, but other surface finishes should be able to be used, again assuming chemical compatibility.
Potential Problems
When combining different finishes into a finishing system, there are three main areas of concern: chemical compatibility, intercoat adhesion and final appearance/color.
Chemical Compatibility
As mentioned a few times in the above section, you need to take the chemistries of each of your coating systems into account. This is mostly referring to carrier systems for each of the different products and how they will affect the previous coating. Aside from 100% pure oils, all coatings usually have some form of carrier solvent. This is water for waterbased products, alcohol for shellac, lacquer thinner (polar solvents) for lacquer and non-polar (oil-based, organic) solvents for most oil products and varnishes. When layering products, it is important to consider the order that you do so and to think about which solvents will come into contact with the previous coat and where this can be a problem. For instance, it would generally be OK to put oil and waterbased finishes over lacquer or dewaxed shellac, however, the alcohol and solvents used in shellac/lacquer can harm oil/water finishes and cause drying issues, or coating failure. Most oil and waterbased surface film-forming products play well with one another as long as each one is thoroughly dry before coating. For example, it is common for people to use 1-2 coats of Waterlox Original Sealer & Semi-Gloss Finish for the look and color and then top coat with Oil Modified Urethane or other waterbased urethanes to get more plastic like durability.
Intercoat Adhesion
When finishing a project, the aim is to form a complete layering system to provide the look and protection that is desired. If your chemistries are compatible and you are not harming the coat underneath, you may achieve a very nice looking film finish, but there is a second piece to the puzzle. It is critical that the separate layers adhere to one another! Intercoat adhesion is how well one layer is connected to the layer underneath. Poor intercoat adhesion can result in one layer of finish peeling or flaking off from the layers underneath (see figure 1). It is not uncommon to see this under tables and chairs in restaurants where it may look like small sections of the top finish is missing. This is generally only an issue with surface film-forming finishes as they will form continuous layers.
Delamination: Top layer is not well adhered to coating underneath.
Again, the finished result may appear absolutely fine, but if any layer isn’t adhering well to the layer underneath then you have the potential for a problem. Using the same products over themselves does not generally create a problem, i.e. shellac will stick to shellac, lacquer to lacquer, Waterlox to Waterlox, however some finishes (like urethanes) may need help to adhere. Also mixing and matching different finishes may also require some intervention to ensure good adhesion between products. Other combinations may result in layers that will never have good adhesion. It is ALWAYS a good idea to test your whole layering system before finishing your entire project. The standard test for adhesion is a cross-hatch test and you can check our guide for that specifically for more information.
Adhesion comes in two forms, chemical and mechanical. Products like Waterlox Original/Marine, shellac and lacquer will chemically bond to themselves without additional help. The coatings will slightly dissolve the coat underneath or remain chemically active enough to fully cross-link between the two layers. Alternatively, most urethanes will have special recoating instructions to ensure good adhesion. Today’s urethanes will usually have a certain time frame in which they are chemically active enough to bond to the next layer without sanding which, when exceeded, will require sanding, screening, or buffing to create a surface profile to help the layers mechanically bond to one another. When mixing and matching different coating types, it is important to check for adhesion between the two layers. Every manufacturer is a little different, so there is not necessarily a general guide as to when you need to sand and when you don’t.
Wax
Wax is a somewhat odd product that sits on the surface, yet will not dry into a hard film coating. Kind of like the opposite of thinning a surface finish to make it penetrate, wax is a penetrating finish that sits on the surface. In general, most things do not stick very well to wax, which is part of why it is used as a coating. However, coating over wax is almost always a bad idea. Wax creates a water repellant surface, but it forms a loosely connected layer that doesn’t really give anything for the next coat to adhere to properly. Products like hardwax oils (Osmo, Rubio, Fiddes, etc.), waxed shellac, and butcher block conditioners will cause problems when trying to apply another coat over the surface. The wax needs to be removed to ensure proper adhesion. Wax is usually not a concern when used as the top layer, however, it will need to be removed if a recoat is needed.
Final Appearance
Last, but certainly not least, is the final appearance of your project. Coating over a previously finished surface with something different may greatly change the final appearance of your project. Again, proper testing with scraps or inconspicuous areas is the best way to see what may happen when the full project is completed. The two biggest changes will most likely be gloss level and color.
