Wood – When can I seal the picnic table I made from pressure treated lumber

I don't know about the cedar, but I built a screened porch a few years ago using standard PT lumber for posts and railings and painted it with a coat of primer and a coat of exterior latex pretty much as soon as it no longer appeared damp. Of course the 6X6 posts had been up and drying for a while at that point, probably 6 months or so. The railings were installed last and were probably only up for 2 or 3 weeks when they were painted. When we moved from that house earlier this year there were no problems with the paint, and that was after about 2 years.

Interestingly, the only paint-related issue we had was some peeling on the existing deck railings, about 4 years old and never painted at that point, that I painted at the same time to match the new railings around the porch. I have no idea what caused it, but I can say that I didn't prepare this older wood at all and it probably could have used at least a good pressure washing prior to painting.

I disagree with @Mike - I don't think sealing it will have much effect on the stain. Two reasons:

1. Deck stains are typically (I'm tempted to say always) pigment stains rather than dye stains. Those deposit pigment on the surface of the wood and penetration is minimal. A sealant shouldn't interfere with that.
2. A sealant will typically be a penetrating sealant not a film sealant, so it shouldn't interfere at all with the adherence of the later stain.

That said, I'm not sure you'll see much benefit from the sealant. Penetrating sealants will certainly give you a bit more time to clean up a spill but they won't prevent a stain: that's why tabletops are usually finished with a film finish, and if they're not, you use coasters and place mats. You wouldn't want to use a film finish to a deck because it will certainly peel in short order as the wood expands and contracts under rain and sunshine.

I wouldn't bother. If food gets spilled and you're worried, scrub right away with soap and water. I'll wager you won't notice any minor food marks after the stain has been applied.