An Email Conversation - Does Tung Oil breathe better than a Poly Finish? 

> Question
> I have a timber house with 150mm baltic floor boards and a badly ventilated (poor extension work) sub floor area with variable moisture content. Would tung oil let the timber breathe and thereby reduce the risk of cupping which happened when I covered one room with precoated solid timber boards?
> Also should all coats be diluted at 50% and what is the coverage rate?
> Thank you for your advice.
> Regards

Response
General comments on thinning, coverage and technique are here
http://www.thewoodworks.com.au/technical-data-sheets/tung-oil-aka-china-wood-oil
In theory a tung treated stick is probably going to breathe a bit better than one with a heavy membrane coating - e.g. polyurethane - on one face only. There are many other variables one of which you point out is an adequately ventilated subfloor.
I doubt the pre-coated boards were coated on both faces?
Coating one face is asking for trouble especially if there isn't adequate clearance and ventilation because one face is more open than the other to daily and seasonal moisture variations and hence cupping.
In an ideal world there is adequate ventilation, your boards are acclimatised to the location before laying (they are at equilibrium moisture content, EMC), your boards are all quarter sawn (about 3 to 4 times more stable than flatsawn which is what 150mm baltic pine boards will be), you have a species suited to use as exposed flooring (baltic is/was an average species widely used when carpet or lino was always intended) and you can get a coat on both faces.
If you are thinking of making a business of this get a copy of Bootle
http://www.thewoodworks.com.au/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=626&category_id=65&option=com_virtuemart&Itemid=91
I suggest you try to deal with the subfloor ventilation issue as a priority.  There are health issues here, my son specialises in subfloor ventilation and is full of anecdotes about the mould  & mushroom fields he encounters.
Best wishes
Mike