I've recently built a standing desk out of sandply with a mahogany veneer.
I have stained it, and two weeks ago added a polyurethane clear coat to protect the stain. However, after doing this, the desk is slightly tacky, and things don't slide very well on it.
What can I do to reduce this "stickiness"?
The poly simply needs to cure for a while. Even after it "dries" and is safe to work on, the solvents that keep the polymers in suspension aren't all gone; they'll continue to evaporate, and the clear coat will fully harden in time. Check the can for your particular product; it should have a full cure time. It could take up to a month if you laid on a really thick coat and it's humid in your area.
Contrary to intuition, you can wax furniture that has a poly coat, much like you can wax a car that has a clearcoat. Just pick up a can of Minwax, apply and buff, then let dry overnight. This additional protective coating will be temporary, but hard, water-repellent and slick.
Poly has a naturally rubbery texture especially if you have a thick coat. You might try sanding with high grit sandpaper to break it up.
Alternately you can put another layer of finish on--something that dries to a hard coat, like shellac.
Oil based polyurethane "dries" in two stages. First the solvents evaporate leaving the resin behind. This normally takes on the order of hours, but as others have mentioned it depends on the temperature, humidity, and thickness of the finish. When the solvent has evaporated the finish will still be sticky. The second stage of drying is cross-linking, where the molecules react in the presence of oxygen to bond with one another. Cross-linking is what makes polyurethane finishes more durable than other finishes. It takes a little longer, but it should not take weeks if the finish was good and properly mixed.
I don't think you have much to lose by trying to seal it with shellac, or even wax, but if that doesn't work then you are left with stripping or sanding back to the wood and refinishing - or put a sheet of glass over it.
Checklist for the refinishing:
Try to wipe over with water in which a little liquid soap has been added. Subsequently keep wiping with clean water till the coat has no more soap. There is a slight loss of gloss but no more tackiness. Coat with a thin layer of wax after that.
Use a little mineral oil and let it stay for a short while. Wipe off with clean cloth till most of the oil has been removed and your work doesn't feel oily to the touch.
You applied an oil base finish on top of a water base finish. It won't dry. A fix trick is apply another coat of water base finish on top of it.
