How to choose a finish
Selecting and applying a finish to a newly completed project is likely the most feared and misunderstood aspect of woodworking. If this step goes poorly, the entire project might be ruined. If it goes well, it’s the first thing everyone will notice each and every time they see your project. Wood finishing is likely the biggest single step in furniture making. Stack the deck in your favor and understand what the reasons are for choosing a wood finish.
Weather Wreaks Havoc
Sun and rain both punish a finish. An exterior film finish looks great when new, but needs to be maintained, and that's not always easy. A exterior penetrating finish looks different than a film finish, but has less maintenance.
Protect From the Elements
A quality exterior finish will do a great job at protecting wood for rot and discolouration when out of doors.
Always Test First
Unless you've used a specific finish on a specific species of wood it's always a good idea to apply some of that finish to a sample panel before committing that finish to the project.
Light Darkens a Finish
This center section of the underside of this elliptical coffee table was covered for the last 10 years. The perimeter was exposed and is now much darker. This shows how much wood and finishes – especially oil-based finishes – can darken with age. Even water-based finishes darken over time, though not as much as oil-based finishes.
Most finishes can be applied any way you'd like, but some lend themselves to being applied one way. What skills and equipment you have might affect the finish you choose.
A Quick Finish
Padded on shellac dries quickly, which makes it great for projects that need to get finished quickly.
A Safe Finish
Using a finish that is as safe as possible is an issue for some people, depending on what project they're finishing.
Add Some Colour
Some woodworkers do their best to avoid adding artificial colour to wood, while others often opt to add a stain, glaze or shading to the piece. There's no right or wrong answer, so feel free to do what you like best.
How Much Shine?
Some topcoats have a higher sheen than others. You can often select the level of sheen you would like. You can always rub out a top coat to increase the sheen, or rub it with steel wool and wax to adjust the sheen.
It's Up to You
Brown chose a water-based topcoat for this piece, as he wanted to keep the bird's eye maple as light as possible. He also wanted to ensure the wenge used in the piece would look nice, so he made a finishing panel before proceeding to test how the two species looked with the same finish.
How to Treat Figure
Brown opted for a simple shellac clear coat on this curly sycamore panel, but someone else might have selected a stain to further accentuate the curly figure in this panel. There's no definitively right or wrong answer, so do whatever you feel is best.
Pros and cons
Everything has pros and cons, and that’s the basis for this discussion on choosing a finish. The demands a piece of furniture or woodwork puts on a finish are going to be slightly different with each piece. Will the chair live indoors, or out? Will the table be in a high-traffic area, or a quiet dining room? Do you have the ability to spray a finish, or not? Will the finish provide the look you’re wanting on the wood you’ve used? These questions, and many more, need to be answered before you can make a final decision on what finish to use.
Sometimes the decision is easy, and other times there won’t be a clear winner, but at least being familiar with this process will give you a way to sort through all the finishes on the market. I think many woodworkers apply the same finish to every project they complete, mainly because they’re familiar with how to apply it properly and what the end result will look like. That doesn’t mean it’s the best finish for the job, though.
Indoors vs. outdoors
Let’s start this off with a fairly easy one. If the piece will be outdoors you’ll need to use an exterior finish. An exterior film finish is more flexible than an interior film finish, and will more easily move with the wood during constant changes in humidity. You can also apply an exterior finish that isn’t a film finish. Many different options are available that will stand up to the elements quite well.
There’s nothing wrong with using an exterior finish on a piece of furniture that will remain indoors, but the other way around is often a recipe for disaster.
Level of protection
A table in a busy family kitchen is going to have just about everything thrown at it. A finish that will at least stand up against abrasion, moisture and heat can offer a decent amount of protection to a kitchen table. Having said that, you may very well still wear through the finish in a matter of a decade or so, then have to reapply it. Polyurethane is a great finish for this situation, as is a traditional varnish.
On the other hand, if the piece of furniture will remain in a seldom-used dining room, protection isn’t going to be the main concern. You can focus on the other characteristics you want out of a finish.
Also, some finishes will protect against things like chemicals and water, so if you have a very specific and unique use for the piece you’re finishing you should look into what will best help you choose and apply a lasting finish.
Finishes look different depending on the species of wood they’re applied to. I find one clear topcoat may muddy the look of a wood, while appearing quite clear on another wood. A different clear topcoat may cause one type of wood to appear too dark the moment it’s applied, while another wood won’t look any darker. Make a sample panel and test out a finish on the species you’re using before deciding what finish you’ll use.
A film finish is one that builds up a layer – or film – on the wood and offers a thick look similar to a thin layer of clear plastic. Some like the look of a thick film finish, while others can’t stand it.
Some finishes change colour much more than others. Oil-based finishes tend to yellow a lot more than water-based finishes, and that should be factored into the decision. Keep in mind that an oil-based varnish, for example, yellows as time goes on. The look you’ll get after a day of curing is going to be quite a bit different than the look after a few months, and again after a few years. Remove a tabletop that was finished with an oil-based product after 5 years of use. You’ll clearly see where UV rays never got to the area that was covered by the aprons, and will be a lot lighter in colour.
All finishes can be applied by any method, but some tend to lend themselves to a certain type of application a bit more than others. An oil/varnish mixture could be sprayed on, but it will still have to be wiped in, and the amount of excess finish that would have to be removed after spraying it on could be substantial. Lacquer is usually sprayed on, as it tends to dry quickly. Brushing lacquer onto a large panel will often result in a slightly uneven finish, unless you have extensive experience with this sort of an approach. A film finish is usually brushed or sprayed, as it would take far too many wiped-on coats to form a proper film.
Dealing with sawdust
Some would say to only finish in a dust free environment, and I can see their point. The problem is, in the real world we don’t all have space or money for a dedicated finishing area to ensure dust-free application, and applying a finish in the same space we create sawdust is our only option. Cleaning the area is a good first step, but also selecting a finish that can handle a bit of dust is also a smart idea. A thick coat of sprayed-on polyurethane will take a long time to dry, and dust will have what seems like eons to settle on it before the dust doesn’t settle in the wet finish. On the other hand, a wipe-on oil/varnish mixture will go on much thinner, and if any dust settles on it before it cures it can likely be removed quite easily.
Creating a more balanced panel
Applying a finish to both sides of the wood, in order to decrease moisture transfer, is a good idea. This approach by no means locks the piece of wood in place, but it does slow the transfer of moisture into, and out of, the wood, minimizing seasonal wood movement. As a general rule a thicker finish does this best, but if minimizing the amount of wood movement is critical to a project you’re working on, you should do some research into what types of finish are best for minimizing moisture transfer.
Are you in a rush?
It’s Christmas Eve and the only thing stopping you from going to bed is the fact that you have to apply a finish to the picture frame you’re making for your spouse. What type of finish do you use? One that dries fast. Shellac dries quite fast. Water-based polyurethane dries a lot quicker than oil-based polyurethane, but you still likely won’t be able to get three coats on in an evening. How fast a finish dries is rarely critical, but if it is you’ll want to be familiar with a number of different finishes. Generally speaking, stay away from oil-based products, as they dry slowly.
Food-safe finishes may need to be considered for items that will come in contact with food. I’ve heard that once a finish is fully cured, all finishes are safe for food, though some people will want the added insurance of using a finish that has never had anything harmful in it.
Safe application is another thing. Some finishes contain harmful chemicals and may irritate your skin or lungs during the finishing process. Using low-VOC finishes is also something to consider. Whether it’s during the application process, or while a finish is off gassing after it’s applied, a low-VOC finish will be safer to have in your shop and home.
Staining, glazing and shading
With so many options for adding colour to wood, I’ll only briefly cover this aspect of finishing. A stain is applied directly to the wood. A glaze is applied between coats of finish. Shading is done by adding a colour to the topcoat before you apply it. The choice of which, if any, of these steps to incorporate into your finishing schedule is strictly a matter of colouring the wood so it looks good to you. There is no benefit beyond that.
Very much a personal choice. If you want a high sheen you will likely have to select a film finish, and ensure the pores of all porous or semi-porous woods are filled, in order to be pleased with the result. Although it’s not necessary, finishes with a high sheen are usually sprayed on.
I’ve briefly mentioned this a few times above. You might prefer the feel of one finish to another. Or maybe it’s the colour of a finish you prefer. Listen to your gut when you have a preference, as long as you’re not overlooking an important factor. Once the more scientific metrics like abrasion resistance and interior vs. exterior have been dealt with, not everyone is going to agree on what the best finish for a specific situation is, and that’s fine.