Try some Spray 9. Let it soak.
Rg
did that.
also Simple Green.
I am tempted to use a light bleach solution. but it will lighten the deck board.
The deck is pressure treat wood.
Never sealed.
Never stained.
We wanted a barnwood type finish.
And it worked and we think it looks great.
Except this long 5-6' grease stain.
lol
here's a couple of old photos of the inside of the deck from a turkey fry we did back on 09
And one from WAY back when I finished building it in 03
today it looks like this -