Doug, I knew that the resin needed a sealed bed, so I used a layer of latex (wallpaint) to achieve that.
Si, actually there is some water on the floor. But I had some newspaper there, so not a big issue.
Also I had made some small dams for compartments, also on the baseboard edge, so the water was not running down immediately.
Not sure about the wood. I have a few different types, depending on what is available in the right size.
Best guess: abachi wood for the planks and beech for the underframe.
The grain looks good in the photo but remember that this is 1:87.
When I look at the wood that I have laying around then I think that it is not course enough for what you want.
So the painting. I used watercolor.
Some brown, some black and that toned with a bit of white. Dilute it with plenty of water so that the wood don't become too black.
After that dried the wheatering is done with powder. A kind of dust color.
The roof is made of filling paper with came with a package. The structure is nice.
I mixed some glue (original tacky glue) with some black watercolor and water on some hard flat plastic.
Insert one strip each time and dab dry immediately (on a tissue/ kitchen paper) so that some of the paint mixture goes out.
That gave the effect of old tarpaper.
I presume the "dusty" lower areas represents the dried limewash used to disinfect such vehicles,
well, at least that was the case in the UK in the early 20th century although I think a true disinfectant began to be used in the late 1920s.
Anyway, whatever the cause it's a first class effect.