I need advice from you woodworking pros.
We bought one of those kits, where all you do is add the 2x4s to make a
6-foot wide outdoor bench that converts to half a picnic table. Because the bench would be outside all year round, I originally put it together using treated 2x4s. Not being an experienced woodworker, I didn't realize that once the treated wood dried out, the treated 2x4s would end up all twisted and warped because the supports on this bench are about 5 feet apart, so the boards span about 5 feet.
Now we're going to replace the treated 2x4s with untreated 2x4s. I was considering buying cedar 2x4s, which we would varnish or polyurethane so the cedar doesn't turn gray. However, cedar is pretty expensive, and someone told me that as long as you're going to varnish or polyurethane the wood, you might as well save money and use regular, untreated pine 2x4s. He said, as long as you varnish or polyurethant the wood once a year, it shouldn't rot from being outside. (BTW, the wood does not contact the ground on this bench.)
I know that, theoretically, that should be the case. But will the bench really last as long as if I used cedar? Has anyone built outdoor furniture from pine ... and if so, how long does it last? If pine will work fine, I'd prefer that because cedar costs, what, 3X more?
Any advice would be appreciated!! Also, are there any tricks to working with treated wood, so it doesn't twist and warp when it dries out? From my experience, it seems treated boards are only suitable when you're screwing it down every 16 inches or so, like on a deck. But maybe there are tricks I don't know of.