I have tons of problems with the whole "ark" story, but coating the
bottom of the ark is not a problem. I do not believe it was ever built,
but my answer will be to the way it could have been done (building a
giant box only, loading all the animals and caring for them is impossible).
There is no way they would have built the bottom inside a frame, off the ground.
Too big, too heavy, too much time. I imagine they would have coated and dried
each piece of lumber before placing it on the level ground. After attaching them
to each other, they could coat the top again. There would be more than one layer
for strength, so the final layer wouldn't need coating (at least on the top).
WTS literature really makes it understandable that such a giant box was feasible.
They try to distract us from the logistics problem of squeezing all the animals and their
food inside the box. Even if weined baby elephants were used, there are millions of
insects, there is dung removal and ventilation problems. There are plants that would
not survive the flood. I could go on- the point is that it could not have happened.
WTS does make it clear that such a box of the size listed in the Bible could have been
built, though. I believe it would have floated, too, and been properly weatherized.