The Paint Manufacturer's Mystery

A. has been asking me questions about paint lately. "What is paint made of?" "Who first made paint?" etc. Last night, after he was supposed to be in bed, he drew a mock-up of his new invention: A paint machine.
I don't know if you can read the typing (done by me, not him, BTW, although he did label the machine "Pait") - over on the left are the chambers for the ingredients, on the top is a vent for steam, the broccoli-shaped thing is a hand that does the work inside the machine, and the square with the little squares inside is the screen to talk into. The paint comes out the right side.

So this morning he wanted to actually build his creation. He told G., "The things I need are: plastic, wood and some flat bed trailers."

They went out to the garage and got some shoeboxes, plywood and duct tape. G. also attached a mini tape recorder to talk into.

"It's not exactly the same," complained A.

"What else does it need?" I asked.

"It needs a hand inside."

"OK, so we'll figure out how to add a hand."

"Yeah!"

The thing is, when it's done, I think he believes it will really make paint.

After the building frenzy, we drove through a very nice neighborhood to a very nice park. A. found it puzzling, though, that some of the houses were made of plain wood shingles and not painted. To further his distress, the playground equipment was bare wood, also unpainted.
I tried to explain that sometimes people like the look of wood itself and leave it unpainted on purpose.

"But WHY??"