When ever I prep a deck for restaining, I clean it with a mixture of TSP, bleach and water. I apply the mixture, scrub it with a stiff broom and rinse it well with water or a power washer. Let it dry well, then apply stain. The cleaning solution helps remove the dirt, grime, spills, etc. Pretty basic stuff, I admit.
As far as why your stain doesn't want to dry. There may be several reasons for this. First thing that comes to mind is that the stain may have not been mixed well before application. The solvents and drying agents can separate from the oils and pigments in stain. The oil tends to stay at the top of the can, and the oil won't dry well without the solvents mixed in.
Another reason may be other sealers, grease or spills that have sealed the skin of the wood, not allowing the stain to penetrate.
Sometimes, one can get a little carried away and apply way too much product. This will saturate the wood and the access will sit on the top of the surface.
If after a couple weeks of good drying weather, you still have sticky or tacky spots, I'd be tempted to remove this excess with a cloth wetted with mineral spirits. This should thin the sticky stain, remove some and distribute it to help it dry.
Maybe too late to ask, but, are you using latex or oil based? If you are using latex, if you can get it on for 4 to 6 hours before a light rain, it will have dried enough to survive without spotting. Oil based really needs 12 to 24 hours to set up depending on the temp. Curing time is a lot longer, but the skim drying is much faster for latex than oil. Good Luck!
Best Answer
Prep is always the most important step. Make sure the wood is dry. It shouldn't make too much of a difference that it rained after you sanded it down as long as it still has pores to absorb the stain.
I always recommend placing a large tarp over the area so that if it does rain, you don't have to worry too much about your work.