We love our hinoki tub that you built and that was installed in our home about a year ago.
The tub itself is fine, but the wooden slats on top have developed mold. Is there anything we can do to get rid of it?
For the mold stains, try to use some bleach, it should become clean again. -> http://en.wikipedia.org/wiki/Bleach
1) apply a small amount of bleach on a dump cloth and friction the mold spots. An old bath towel should be fine.
2) wipe immediately with a clean dump cloth.
3) repeat if necessary
4) rinse abundantly with the shower while wiping with a cloth to remove any bleach left.
This treatment does not harm the wood if you do it few times a year.
Also, I recommend to turn over (upside-down) the planks from time to time. This prevents the formation of mold and warping of the planks overtime.
In case you do not want to use the bleach or if it has scarce effect, the standard solution would be to use a planer. Also fine sand paper will do the job.
Let the wood dry well and use first fine paper (#240) to remove the wood skin. Finish with very fine paper (#400). Reember to follow the direction of the wood veins.
Please let me know if it worked out.