surface treatment for a japanese tub

I am glad you found a good solution for the tub.
If you apply a coat on the wood note the following:
- The tub will loose its natural "soft" touch
- The tub will loose its aroma and its skin-soothing properties
- You would have to seal it inside and outside (including bottomside) to prevent unbalanced behaviour
- Due to the oils contained in the wood, the coat may not stick uniformly and may detach. Consult also with the maker of the coating product.

Generally we recommend to use it untreated or apply a natural oil to hydratate the surface of the wood.

Additionally there are also some non chemical clear wood treatments which last one year and provide extra protection against mold.

Please let me know if you need some product.
If you have a specific problem, please send pictures of the tub and the installation conditions.

Thank you. If I do not treat it, will it deteriorate more quickly than if I treat it? I like the way it is but am worried about it deteriorating. I am having a cover made for it to keep it out of the weather. However, it got quit a bit of sun last year and cracked in some places. It does not leak, and the cracks are probably natural and not a problem. I like the way it looks now that it has aged for one year. I just wanted to make sure that I'm not supposed to be treating it with something. If there is a natural oil that I should apply, please let me know.

It is not necessary to treat the tub and usually japanese people do not apply anything.
In case you want to restore the original shine you can apply our hiba oil with a cloth, like you would do waxing a pair of leather shoes.

for purchase please refer to:
http://www.bartokdesign.com/japan/2-products/wood_tub_cleanerconditioner.php
and for more specific information to:
http://www.bartokdesign.com/japan/0-blog_news/new_product_hiba-oil.php

Please keep the tub protected from direct sunlight and if you cannot, leave the tub full of water to prevent dehydratation and cracks.