Is it absolutely necessary to paint the sheet of plywood before you do anything else to it? Thanks.
no, if you mean as a base coat for scenery then do that after you have lain your plaster or however you plan on doing scenery. But no, you dont have to paint it.
As long as you don’t get the plywood too wet while making your scenery, there should be no problem and no reason to paint it first. Most plywood does not expand or contract with changes in the weather as much as pine boards do because of the way plywood is made with alternating directions of the wood grain. Of course, if you saturate it over an extended period of time, it will swell and the laminations will separate.
I like putting a coat of the ‘base’ scenery color on my plywood because it just kinda looks better than bare plywood until the scenery is built. As the others here said, it’s not necessary, but with at least another 3-4 months of work until the track work is finished and the scenery started, I’d rather look at bare ‘earth’ than bare plywood as I test-run trains.
[:)]
As mentioned above, no. But I do it for two reasons and I do it before laying it down if I can, 1) it alludes to a finished look and 2) it helps keep things picked up. I paint both sides, the top is earth tone and the bottom is white. Painting the bottom helps with lighting when you crawl under to string wires or other work.