If the Liquid Restores the Color
All you need to do is apply a clear finish. Your choices are oil (boiled linseed oil, Danish oil or antique oil), shellac or varnish. The differences are as follows:
Oil will penetrate deeper because it cures slowly, so it will make the wood darker than the other two finishes. Moreover, the color will continue to darken some as the oil ages. If the color produced by the test liquid is just a little light, oil might be the best choice.
Shellac dries very rapidly, so it doesn’t penetrate as deeply or darken the wood as much. Clear shellac is probably what you should use, not amber, and you may want to apply the shellac with a fine artist’s brush depending on the size of the damage.
Varnish darkens more than shellac but less than oil, and it also darkens a little as it ages. An artist’s brush is also useful for applying varnish to small areas.
If the Liquid Doesn’t Darken Enough
You need to apply a stain, and as long as you use an oil-based wiping stain or a water-soluble dye stain, you can simply wipe the stain over the damage and then wipe off all the excess. With this method you won’t leave any mark on the surrounding finish.
Choose between the two types of stain based on how much color you’re going to need to add. Wiping stains won’t add as much color as dye stains will, and you can continue to put more dye onto the damage and make it darker, while wiping stain will have little additional effect after the first application.
If you use a water-soluble dye stain on a tabletop, you should seal it in by wiping over with an oil finish so it doesn’t get washed out during cleaning and dusting.
Instead of using one of these stains, which gives you a lot of control of the color because there are infinite possibilities, you could use a commercial product designed for just such a problem. These include Howard’s Restor-a-Finish and a number of brands of colored paste wax. Or you could use a touch-up marker, which is especially effective on sharp edges.
If the Liquid Has No Effect
If the liquid you applied in your initial test doesn’t darken the damaged area at all, the wood is still sealed with finish. A lot of factory furniture is sealed first and then colored with toners and glazes. This coloring could have been removed without breaking through the sealer coat.
To reintroduce color to this type of damage, you have to paint it in. You can do this with a touch-up marker, or you can brush on any colorant that includes a binder. The binder should be shellac, varnish or water-based finish. In effect, you are painting with thinned paint. The trick is to get the color right, so standard paints seldom work. You need to mix up the colorant and binder yourself. Use Universal Tinting Colorants (UTCs) with shellac and water-based finish, and oil or Japan colorants with varnish.
If the Liquid Makes the Color Too Dark
This scenario usually indicates that the wood has been roughened, and too much of the liquid is retained. There are three possible finishes you could apply to get the color right. Clear paste wax will darken the least. Water-based finish will cause some darkening. A fast-drying finish like shellac won’t penetrate much, so it will also cause very little darkening – especially if very little is applied.
This type of damage is actually the most difficult of the four to get right, because you can always add more color to make a repair darker, but you can’t make it lighter.
Of course, experience is always helpful for achieving success, but the first step, even if you have a lot of experience, is doing the liquid test. PW