Get a cleaner that contains oxalic acid or sodium percarbonate, there are plenty of good ones out there, the company I work for sells the Wolmans brand. Scrub it down real good with extra emphasis on nooks and crannies. Do small sections at a time. Follow the directions on the cleaner, that's what they're there for. Rinse it after prescribed time, when you think it's done rinse it again. Let the deck sit for two nice sunny days. All old stains and finishes should have been removed. Splash some water or thinner on your deck before staining. If it beads up anywhere you need to do more work because the stain needs to penetrate the surface of the wood
I always recommend semi transparent deck stain over sealer, clear sealer just don't last on exterior applications. Make sure it says for decks. Avoid the extra thick stains like Restores made for questionable wood. If you have a rotten board replace it, the thick type deck finishes have been nothing but problems since they have come out. Latex deck stain isn't as clear, it's more like thinned down latex paint. Latex deck stains tend to look a little blotchy but the great thing about latex deck stain other than easy clean up is it won't mildew near as bad as oil based deck stain. Oil deck stain will show the beauty of the wood since it is a clearer finish. If mildew grows on your house you have to realize it will grow on your deck but to me a deck looks far better when stained with semi transparent oil. You can use a brush but a pump up garden sprayer or an airless sprayer is what I would use. Keep a brush handy to brush out puddles and to just even things out. Never stop staining in the middle of a board, you always stop staining at a place where you have a natural break. Before you apply a second coat read the directions, some stains companies don't recommend a second coat or have a specific time for applying one. One of the worst things you can do is put on too much stain. The absolute best stain for decks is made by TWP. Others are decent but TWP has impressed me. Avoid Sikkens as it is far too expensive even though I've never seen a clearer, cleaner looking finish.
No finish is needed over a good deck stain, usually they contain linseed oil or something similar that will act as a sealer. The absolute worst thing you can do is apply a varnish over an exterior stain, just leave it alone when your through staining.
PM me if you have any questions, I have 42 years in the paint business, 31 years with the same company.
