Looks like some good progress is being made!
What size Engel piston are you using?
The decks on a Type VII, though mostly wood (metal at very bow and stern), were usually not seen in their natural color. A wood preservative was used that, when applied, was initially just about a black color. The effects of sun and the sea would quickly cause it to fade to a dark gray. Boats coming back from long patrols could be seen with decks that were a lighter gray yet and would finally show the slightest bit of brown.
-hope this helps!