You may want to bleach your old knotty pine cabinets or paneling to lighten them, or you may want to remove a particular type of stain; use a different type of bleach for each of these purposes. Bleaching can be a prelude to spectacular whitewash or stain treatment that will revolutionize the appearance of your kitchen or living room. Keep in mind that wood bleaches are caustic and must be handled with respect.
A Purpose for Every Bleach
Ultraviolet light from the sun can do a pretty good job of bleaching the natural colors from pine, but there's a faster way to do it that doesn't leave a gray surface coating of dead wood cells. A two-part bleach consisting of sodium hydroxide and hydrogen peroxide does just that, and you can apply it more than once to make the wood progressively lighter. If you're more concerned about removing dyes left from juice spills or ink, then you need chlorine bleach. Use oxalic acid to bleach out dark stains created by reactions between the wood and minerals in water that have penetrated the grain. Oxalic acid also restores the natural color to wood that has turned gray from exposure to sunlight.
Preparing the Wood
You can only bleach unfinished wood, so if you want to lighten your wood paneling or cabinets, you probably have an extensive stripping and sanding session in store for you. Dismantle as much of the woodwork as you can to make this job easier; if you're bleaching cabinets, remove the doors and all hardware, and if you're bleaching walls, take down all the trim. Rinse well after stripping, and let the wood dry before you sand. Knotty pine paneling frequently has deep grooves, and the fastest way to remove the finish from them is to sand them with a rotary tool and a detailing brush, which looks like a paddle wheel made of sandpaper.
Begin by Removing Stains
The wood may not have dye or water stains, but if it does, it's best to bleach them out before lightening the wood color. Mix a saturated solution of oxalic acid crystals and water and paint this solution on dark wood stains and gray wood; be sure to wear gloves and goggles when using this or any other bleach. Once the stains or the grayish tint is gone, neutralize the acid by rinsing with a baking soda solution. To remove dye stains, brush or spray liquid chlorine bleach on the stains and let it dry. Repeat the procedure twice more if any color remains; after that, further bleaching will be ineffective.
Lightening the Wood
Two-part wood bleaches for lightening the natural color are readily available -- they consist of a part A -- sodium hydroxide -- and Part B -- hydrogen peroxide. You can sometimes mix the parts before applying the bleach, but it depends on the product, so read the label. If mixing isn't recommended, sponge on part A and follow it immediately with part B; don't wait, or the sodium hydroxide may darken some parts of the wood. You'll probably only need to bleach once, but you can do it again if you want a lighter color. This bleach is alkaline, so wash the wood with a weak acid to neutralize it; a 1-to-2 solution of vinegar and water works well.