Most wooden hot tubs are made from cedar or redwood. Both of these woods resist insects and wood rot, and both stand up well to prolonged contact with water and chemicals. However, regardless of how well your wooden hot tub stands up to the elements, there will come a time when you need to refinish the wood to keep the tub looking and performing its best. The process of caring for hot tub wood is exactly the same as that for preserving a deck made of the same wood.
Drain all of the water from your hot tub. Use a shop vac if necessary to remove the last of the water and wipe the tub down with an absorbent cloth to remove the last traces.
Scrub the wood well with a mixture of 1 gallon of warm water and 1/2 cup of trisodium phosphate (TSP), stirred well to fully dissolve. Dip a nylon brush into the TSP solution and scrub the wood of your tub, paying particular attention to stained or discolored places. Rinse with a hose when done and dry the tub completely.
Wipe a bleach solution onto the wood of your tub to remove any mold and other growth. Mix 1/2 cup of bleach with one gallon of warm water. Wipe this bleach mixture all over your tub with a cloth and allow it to dry. Wear gloves and a breathing mask and be careful not to get any of the bleach mixture on your clothing. Hose the tub off again and dry it thoroughly.
Sand off any old finish on your hot tub wood. Start with 60-grit paper, sanding with the grain. Sand until all old finish is gone and any scratches or gouges are gone or minimized. Wipe the wood off with a cloth.
Continue sanding with 100-grit paper and finish with 180-grit paper, sanding with the grain in the wood until the wood is as smooth as glass. Vacuum the tub with a shop vac to remove as much dust and dirt as possible and wipe the entire tub down with a tack cloth to remove any remaining dust.
Stain the wood. Use a brush or simply dip the corner of a cloth into your wood stain and rub the stain over small sections of the tub at a time. Allow the stain to sit on the wood for one minute and then wipe the stain off with a clean cloth. If the wood is stained too lightly, leave the stain sitting on the wood for a longer time before wiping it off.
Apply two coats of outdoor polyurethane to your tub, allowing the first coat to dry completely before adding the second coat. Use a brush or a roller to apply the polyurethane. You may wish to lightly sand the first coat of polyurethane with your 180-grit paper, sanding just enough to smooth it to a consistency of glass before wiping it off with your tack cloth and applying the second coat. Allow the polyurethane to dry completely as per the manufacturer's instructions before refilling your hot tub.