1) Generally better to use a stain that soaks into the wood. If peeling, need to remove completely and then clean wood surface, be sure it is dry and then recoat. If very loose paint, may want to wash with strong garden hose to knock out as much peeling paint as possible. Then dry. Then sanding probably quickest.

2) Spindles are a great deal of work to strip. Chemical stripper could be used for surfaces if not flat, but this is almost as much work as replacing railing. A light coat will not cover over dark paint. Will most likely take several coats. You also need to decide if existing paint surface is solid enough to paint over. It would be really annoying to paint all railings and then have them peel in a few years because underlying dark paint starts to peel.

3) Vinyl and composite railing not getting any cheaper. If you are really considering doing this, might as well do it now and save all the work of refinishing the rails. Alternatively, rails and deck do not need to be the same color. Can the deck be painted white and the rails remain in the existing contrasting color? Another alternative, which I have decided for on my deck, is Aluminum railing. Thinner than composites with a wide range of options from conventional balusters to glass panels to cable railing(quickest to install and never needs painting)

Just a thought regarding white stain - it reveals EVERY little crack. This may or may not matter in your situation.

Our porches and deck had been left untreated in any way for several years before we finally got up the energy to stain them. My first choice for a color combination on our traditionally-styled white house was a grey on the floor boards, and white on spindles and railings. I think this would have looked beautiful had the deck been newly-constructed, but it was a huge mistake on our old-ish deck. Every single crack, big and small, showed up as black lines on the stark white background when stained. This was most apparent on the posts, which had substantial cracks running vertically. If your deck was well-sealed soon after construction, you may not have any cracks. If you do, I would advise against a light-colored opaque stain. I spent hours stripping off the small amount of white stain I had applied, before staining with two coats of a semi-transparent cedar naturaltone (Olympic.) The deck looks great with the cedar naturaltone, despite the cracks.

I also advise against staining white on top of a darker stain. If the old stain is peeling, the new stain will eventually do so, too, revealing the underlying dark stain. (It also may take several coats to cover the darker color.)

If I were in your situation, I'd probably try to fix the current problem, either sanding to remove loose stain and get a nice "tooth" to which new stain (of a similar shade) can adhere, or stripping the deck entirely (by sanding.) But, if you are of a mind to replace aspects of your deck with low-maintenance products, you could do that, too.

Thank you, both of these were very helpful. Lizzie I had not thought about the cracks - yikes - thanks.

Oh, one other thing - I mentioned I ultimately used two coats of stain. A lot of manufacturers say to use only one coat (and that's the usual advice here, too.) I found that for old wood - and maybe this is most relevant only with old unsealed wood - one coat got soaked up immediately. In many areas, the wood ended up looking almost as if I hadn't stained it at all. It definitely needed two coats, for appearance, as well as to get a good seal. (Water beaded more on two coats than on one.) So far our porches, which we did last year, have held up great, even after an incredibly snowy winter with lots of shoveling on the surface. I just made sure to let the first coat "cure" for 24 hours on any surfaces which would get walked on, before adding a second coat. I did the second coat on other areas when they were dry to the touch. No peeling or anything....

Anyway, good luck to you - old decks can be such a pain.