Stop in for a cup of coffee

Mopar Tim said

Make a plywood box large enough to hold one piece or more at a time and line it with plastic, then fill it with preservative, and let lumber soak some. Do all lumber and no more problems with rot or bugs.

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That would work, but till he does all that he'd be further ahead just buying PT. I would just build it with whatever he has. Paint a couple coats of whatever exterior paint is on hand. when it starts to rot you don't need to feel bad scrapping it lol.