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If the wood is unfinished, which means it is its original color, then you'll simply prepare the wood and add stain. If it is already stained you have a decision to make -- either stain over the old color or remove the stain and start over.  If there is a finish on the furniture (a clear coat that protects the wood), you must remove this before starting.  If you're adding a darker stain to a lighter piece, you can generally add this new stain over the old one without having to remove the original color. Stain can change the color of metal handles, knobs, and hinges permanently. They are best removed before beginning. If the metal cannot be removed then use painter's tape to carefully cover it all up. Plastic, glass, or rubber should be similarly removed to prevent accidental discoloration. To get the most even and total coverage you should stain each part of the furniture individually. This prevents missing areas around joints or corners as well as stain pooling up along edges or in cracks. While not strictly necessary, this step will ensure the most professional finish. Work in the direction of the grains to avoid making scratches. This means that you sand parallel to the lines in the wood, not against them. If the piece is large you can use an orbital sander with 120 grit paper to work more quickly.  If the furniture is very worn, go at it with a coarser sandpaper (80 grit or so) until you have it nice and smooth. You can then move to the medium-grit paper. The lower the number on the sandpaper, the coarser (rougher) the grit. Simply wipe off the wood dust as you work, making your sanding easier and more effective. A tack cloth is the best material, as it attracts the wood dust. Sanding opens up microscopic pores in the wood. The stain can then soak into these holes, changing its color permanently.  For very delicate pieces, work up to 220 grit slowly. Go from 150 to 180, then 200, then 220 or higher for a perfect surface. If the furniture is already stained and the color is still there after sanding, you will either need to continue sanding with a coarse-grit paper or use a chemical stripping agent to remove the old color This will bring out the wood's natural color, which will give you a better and richer color once the stain is applied. Simply use a clean rag or sponge to wipe the whole thing with the spirits, then wipe them off with another clean rag. Even the easiest to stain wood -- oak -- could use a little wood conditioner for a better coat. Wood conditioner, which is easily applied with a paintbrush or clean sponge, should be left to dry for 10-15 minutes. It is essential for furniture made from:  Alder Aspen Birch Maple Pine Fraser Ceder Right before you start staining, give everything a quick wipe to prevent any dust or dirt from being accidentally stained over. If the piece is already stained dark black, for example, and you want a honey color, you'll likely be sanding all day just to get started. The alternative is a chemical stripper which, though messy, will remove the majority of coloring. To use one, purchase a stripper labeled "wash away" or "no cleanup," then move your furniture to a well-ventilated area:  Put on gloves and eye protection. Apply a thick coat of chemical stripper on the entirety of the wood. Let the stripper sit according to the manufacturer's instructions. Use a putty knife to scrape away the stripper, working in the direction of the wood grain. Scrub away any last stripper with steel wool. Sand the furniture with a fine-grit paper (200 or higher) once it is dry.
Decide if you want to stain over the current color or start from scratch. Remove or cover up any non-wood hardware on the furniture. Breakdown larger furniture into smaller pieces if possible. Sand the whole piece with medium-grit (100-120) sandpaper. Use a clean cloth to wipe off the dust between sandings. Use a finer-grit sandpaper, around 220 grit, to smooth and polish the surface. Clean the wood with mineral spirits. Apply a wood conditioner for softer or difficult-to-stain woods. Wipe the entire piece clean to remove any last dust or wood conditioner. Consider a chemical stripper of you want to drastically change the furniture's color.