Wood projects which are stained should almost always have a clear finish applied over the stain. About the only exception to this is when exterior oil-based stains are applied to rough cedar siding. The oil in the stains itself is enough of a protect for the wood then, as the cedar is highly resistant to wood-eating insects and fungi caused by rot.
Part of the reason for applying a finish over stain is that stain isn’t actually a finish, but rather something that soaks into the wood to change its color and tone. It does not provide any sheen, does not waterproof the surface of the wood and does not protect the wood from accidental staining by liquids or sticky substances being spilled upon it.
While there are other finishes that can be used on stained wood, the most common is to use polyurethane, commonly referred to as polyurethane varnish. This is not to say that it is actually a varnish. The term “varnish” has become a generic term, used to describe any clear finish applied to wood. However, polyurethane is a water or oil based plastic resin. Varnish, as in old-style varnish, was made from boiling down linseed oil, with other resins and solvents. This is still available today; in the form of marine spar-varnish and some “natural” varnishes. But most of what is called varnish today is actually polyurethane.
Part of the change from old-style varnish to polyurethane is that polyurethane is both easier to make and easier to work with. Polyurethane is less expensive, yet is more scratch resistant than its predecessor. Polyurethane is more flexible than varnish, making it less prone to cracking. It is also more scratch resistant. However, varnish is a better product for using outdoors, where it will be exposed to the elements; hence the popularity of spar varnish on boats, rather than polyurethane.
Staining the Wood
Stains vary considerably from brand to brand and between the different lines each brand has. They can even vary from color to color, as the pigments are made of different materials. So it can be difficult to come up with hard and fast rules for working with any of them. Some stains today are water-based, in an effort to lower the VOCs (volatile organic compounds) for air quality. However, many woodworkers still stick with oil-based stains, partially because they are used to them and don’t like the differences between working with those oil-based stains and the water-based ones. There are also lacquer-based stains on the market, predominantly made by Minwax.
Just as there are different types of stains, there are also different manners of applying them. Some people apply the stain and allow it to dry. This is good if an even finish is desired; but it doesn’t do anything to bring out the grain. To do that, it is necessary to apply the stain and then wipe it off, before it dries. The timing on wiping it off is somewhat critical in that it needs to be consistent over an entire project, to ensure even coloration.
Ideally, any stain should be applied in two coats to help ensure uniformity. However, many stains will provide good coloration with only one coat and manufacturers say that the single coat is fine, as long as the woodworker is satisfied with the results.
Please note that if stain is allowed to dry on the wood, without wiping, it will need to be wiped after drying, before the polyurethane is applied. Polyurethane applied to that heavy coating of stain will not adhere properly to the wood. Therefore, as the stain dries further, it will flake off, taking the polyurethane with it.
If the wood remains tacky or gummy it is a sign that too much stain has been applied and the excess needs to be wiped off.
Applying the Polyurethane
Stained wood needs to be dry before polyurethane is applied. The problem is that not all stains dry at the same rate and there are several factors which will affect how quickly the stain will dry. By and large, water-based stains are ready for polyurethane after 3 hours, oil-based stains are usually ready for polyurethane after 8 hours, and lacquer-based stains need 12 hours before applying polyurethane. Temperature, vent and humidity will all affect drying times.
Before applying the polyurethane check to see that the stain is dry. Polyurethane that is applied to stain that is not dry will not adhere to the wood properly and will end up coming off. It is usually possible to tell if a stain is dry by simply touching the wood. If unsure, try sanding an inconspicuous part of the piece. If sanding dust is produced, the piece is sufficiently dry.
While checking to see if the wood is dry, note how smooth the surface is. Stains will raise the grain in some types of wood, necessitating light sanding with a fine sandpaper before applying the polyurethane. Failure to sand at this time will only mean that the first layer of polyurethane will need to be sanded off to smooth out the surface.
Always clean the dust off of wood surfaces before applying polyurethane. Most woodworkers don’t have a separate finish room, so they end up finishing in their workshops. That’s a dusty environment and the dust will be attracted to the bare wood and then to the polyurethane, once it is applied.
Always stir polyurethane carefully before applying, to mix the solids thoroughly through the solution. Mix slowly and deliberately, so as to not create air bubbles in the polyurethane. Fast mixing will mix air in, causing bubbles in the finish, which will not dry smooth.
Water-based polyurethane is best applied with a synthetic brush and a natural bristle brush is the best for use with oil-based polyurethane. Before dipping the brush in the polyurethane, dip it in mineral spirits, if using an oil-based poly and water if using a water-based poly. Squeeze out the excess water of mineral spirits with a lint-free paper towel or rag.
Apply the polyurethane with long, smooth strokes, striving to make strokes that go from one end of the workpiece to the other, following the grain. Avoid putting on heavy coats, as they tend to sag and run. A better finish can be achieved by using several thin coats of finish.
Each layer of polyurethane needs to be lightly sanded with fine sandpaper (220 grit) to remove dust, runs and spikes in the surface. The idea is to sand the least amount possible, taking off the least amount of finish possible, while still creating a smooth, flat surface. Once the surface is sanded, clean off the dust with a tack rag and apply another coat. Continue applying coats until the desired finish is achieved.