I’m not going to say it depends, that’s punting. I’m going to say every 2 years – for horizontal surfaces anyway. Verticals can go longer, maybe 4-5 years if they’re not south facing, full exposure.
What’s happening on the horizontals is UV damage, rain, snow, foot traffic from people, dogs, and miscellaneous abuse.
Between all of it, it will wear unevenly and require at the very least touching up (which never blends well) but more likely corner to corner refinishing.
Sanding; is it necessary?
Not every time, but from time to time it is. If 2 plus layers of finish are on the surface, the amount of penetration a new coat of stain is likely to achieve is minimal – only in bare spots – and so the stain rests on top, and isn’t penetrating the wood.
For deck floors with transparent or semi-transparent coatings, usually a good thorough rinse will do. Be careful to not power-wash the soft grain out of the wood-leaving a washboard effect. It’s better to use a garden hose and cleanser with a scrub brush fixed to the end of a mop handle. This way clean is achieved without damaging the wood.
This is more troublesome with high-solids finishes like solid deck stain, semi-solid, even some semi-transparent finishes have a higher solids content than others.
A good thing to remember is higher solids=higher build. High solids usually have more protective qualities for typical wear. But, like everything else in life, there’s a trade-off. The trade off with high-pigments is poor touch up characteristics, and substantially more prep when it’s time for redo.
Because of the build up, touching up bare spots results in a mottled look. You can touch up multiple times and that will improve the look but will not blend in perfectly.
This is where sanding becomes important. Rent a floor sander and power through it – remember to check for proud screw or nail heads first, reset them before sanding.
Get as much area sanded as you can with the large rental sander and use a hand held electric sander for the detail areas.
When finished, sweep, vacuum and then rinse the dust off. Let it dry then you can begin re-staining.
My favorite method is using a 9” deck stain pad on a pole – do 2-3 boards at a time and continue along the length of these boards to the end, then move to the next group and continue them to the end. This method takes a little longer but it eliminates unsightly over-lap marks which will show up when the finish dries.
Start farthest in, work your way out to the step/access area. If the steps/access point is in the center, do boards on one side, then the other – leaving the center unfinished – then do the center boards back toward the steps/access; finishing with them.
It’s important to maintain the deck floor & tops of handrails too; keep them clean by washing them a few times a season and the finish will last longer.