While it is true that all wood rots, the softer a wood is, the quicker
it will rot. Remember that rotting wood is giving off pollutants and
while plecos will eat some of the more badly decomposing areas of
wood.... that same wood is still giving off dissolved organic wastes
that the plecos can't consume. If it builds up, it can severely foul
your tank. If the wood is nice and dense, it gives off these wastes at
a much slower rate. I usually don't recommend sealing wood either.
Due to the porosity of most wood, it's almost impossible to acheive a
complete seal with no microscopic gaps in the sealant.... this allows
water in, but doesn't allow for any sort of flow/exchange.... if the
water gets in through a small hole in the urethane and just sits there,
I've found that it causes the wood to putrify and it cultures anaerobic
bacteria. On top of that, I've heard numerous complaints of plecos and
other fish dying from ingesting bits of urethane/lacquer. Grapewood is
a fairly soft wood and I've found that it does not last terribly long
in most any tank before it becomes really soft and spongy.