It's possible to make wood stains by dissolving berries, nut husks and even tea in water, but if you stick with commercial products, you still have a wide array of options. Some stains contain pigments dissolved in a solvent or in water, and others are solvent- or water-based dyes; some semi-solid gel stains behave more like paste than paint. When your stain is too thick, thin it with a compatible solvent.
Know Your Stain
The ingredients in a commercial stain -- including the solvent carrying the pigments or dyes -- are clearly marked on the container. The vast majority of oil-based pigment stains contain mineral spirits, often identified as petroleum distillates. Varnish stains are thicker than conventional oil-based stains, but they also contain mineral spirits, and that's what you use to thin them. Water-based stains tend to be semi-transparent ones for exterior use, and the thinning directions on the can specify water as the solvent. If you aren't sure whether a stain is water-soluble, the word latex anywhere on the label is a dead giveaway that it is. If you're using a lacquer-based or non-grain-raising stain, you'll know it by the strong odor of xylene, toluene or and methyl ethyl ketone -- common ingredients in lacquer thinner.
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Add Thinner Incrementally
If you leave your stain in storage with the lid completely or partially off, you may find a pasty mass in the can when it's time to use it. Most stains -- with the exception of varnish stains -- don't contain curing compounds, and you can make them workable again by adding the appropriate thinner. Add the thinner in small increments, and stir thoroughly until all the pigment or dye is dissolved. It's virtually impossible to make stain too thin to use, but keep in mind that the thinner you make it, the less color it provides. Use mineral spirits for oil-based stains, water for water-based stains, and lacquer thinner for lacquer-based or NGR stains.
Gel stains rarely need thinning, but if you're having trouble getting the stain to spread, you may need to add a small amount of thinner. Check the label to ensure you use the right solvent; some gel stains are water-based and thin with water, while others are oil-based and thin with mineral spirits.
Thinning for Spraying
Most stains are thin enough to spray directly from the container -- just pour some into the cup of your spray gun, adjust the nozzle and spray away. Some semi-transparent stains may need thinning -- but prevent running and dripping by erring on the side of too little thinner instead of too much. You can always add more thinner if the gun clogs during spraying, but it's difficult to thicken stain that you've thinned too much.