I've built a large play structure for my kids and am wondering how (or if) to finish it, as I really don't want to have to refinish it every year or two or three. I used composite lumber for the deck boards and stringers and pressure-treated douglas fir for the structural members.
I'm concerned about the latter. I've seen numerous products advertised on the internet as so-called “lifetime” finishes, but I suspect that these are mostly scams. Are there any real “apply once” products that work, or should I be looking at something like a semi-transparent deck stain? Alternatively, since I live in a moderate California climate, can I just leave it unfinished? I've been told that my kids will outgrow the play structure long before the pressure treated lumber deteriorates.
You're right - your kids will outgrow the structure before the pressure-treated wood deteriorates, but that's not the reason not to finish it. The reason to finish it is the fact that it will crack and get splintery from ultra-violet deterioration from the sun.
To avoid this, stain the play structure using a solid-colored deck stain. I suggest solid over semi-transparent because the more pigment in the stain, the longer it will last. Solid stain looks like paint, but you still see the wood grain through it.