The key to any finishing or refinishing job is proper preparation.
1) Clean the deck, railings etc well, removing as much dirt, grime etc with a power washer on medium setting
2) Wet the area with a mixture of 2/3 gal water, 1/3 gal bleach, and 1/2 cup TSP.
3) Let the mixture set for 5 minutes, then scrub it in with a stiff bristle push broom wetted in the same cleaning solution.
4) BEFORE the cleaning solution dries, rinse it off with a hose or your power washer.
5) Repeat this in any areas that still look bad or stained with BBQ leftovers etc. This should brighten the wood considerably.
6) Repair any loose boards, nails, screws etc.
7) Let the wood dry completely, usually a couple of days under good conditions before staining.
8) I like to apply stain with a roller on a pole or 1 gal pump sprayer, then immediately over brush with a good china bristle brush to even it out. Don't be surprised if it takes two coats to look good again.
9) Don't wait 10 years again to freshen up your deck!!!!! Good Luck
If you must paint this stained wood, a light sanding with 4-O steel wool or 320-grit sandpaper should be fine. All you're doing is roughing up the surface coat, usually poly, which will let the paint "key" to the surface better.
Be aware of the finish originally used on the wood; only poly will take a latex topcoat well, while on most other finishes like varnish or oil-based finishes, latex will not bond to the surface properly; it may peel easily, or even bead up when applied. If this is the case, roughing up the topcoat is not enough; you will have to sand off the topcoat to get to raw wood, and on a profiled wood piece like mouldings, this can be difficult or impossible.
Also be aware of the type of wood; certain woods are "open-grain", and will absorb paint deeply into the structure of the wood, requiring many coats to "fill" the wood and get a solid color on top. Otherwise, the grain and any knots will show through the final coat until you put 3 or 4 thick coats on. If you have to go down to bare wood on an open-grained wood, you have to seal the wood with a product that is compatible with your paint base.
Best Answer
I used poplar in my home, it is cheap compared to other off the shelf materials. It took stain very well, but a word of caution, the heartwood of poplar is green, takes stain well, and when it ages it turns a nice shade of brown from UV exposure. The sapwood is the bad guy here, it is really light in color, as soaks up the stain, so much so, that if there are any dark pigments that make up the stain color, for some reason the sapwood really accentuates the dark color, when on the green heartwood it responds differently to the color of stain being applied, so much it looks like 2 different stains are being used. This problem was not evident when I built my home in 1989, poplar then was pretty much all heartwood. My remodel that I finished last year proved different using poplar. Poplar today has a lot more sapwood, and just a little heartwood which makes it difficult to stain evenly. Maple has the same problem but for a different reason, but the fix is the same. There are "wood conditioners" out there that are applied first to soak up into the softwoods or "curly grain" like maple that will let the stain soak in evenly, at least more evenly than without the conditioner. This may be the simplest fix for the problem, though there are other ways to get past the problems, but they include a lot of trial and error- time consuming.