i use a surface cleaner on trex. No grain to worry about.
penofin.com has a product called KNOT-WOOD
It's for sealing TREX.
The real wood in trex is what get's black and moldy.
I wash it untill it's brown[fresh wood]
Then brightin. Never used the knot-wood.

The more you learn the more you earn! Thanks to everyone for their help.
My motto '' Do it right or dont do it.''