How to fix flaking paint on a deck railing and make it last

if you use one of those powerful floor sanders, be sure to use fine grit paper and go easy. Plywood can damage easily and those sanders are aggressive and designed for hardwood. A good 6 or 7 inch DA sander would be a good bet, especially around the edges. Be careful, don't tear up the plywood with a monster sander!!! lol.

As far as paint, select a good grade floor enamel. Roll it on with a short nap roller. A moehair or 1/4" nap is perfect for leaving a good smooth finish without a lot of orange peel effect.

For the main deck, I would recommend a floor sander, similar to what's used for finishing hardwood floors. Put a 100-120-grit belt on it and take it back and forth along the deck in the same direction as the planks until you see bare wood. Once you have bare wood, I would apply whatever stain or sealant you want very quickly thereafter, as you have just removed a large portion of both the artificial and natural weather and bug resistance the wood had.

A handheld orbital or belt sander will get most of the stuff off the vertical components (handrails and pickets). You'll need a "corner cat" or other detail sander, or some elbow grease, to get the paint out of crevices and detail work.

You will not be able to avoid taking off some wood along with the paint. Paint (especially a product designed for this kind of outdoor application) will seep into the grain of the wood, and you will be removing wood before you see the color disappear and bare wood show through.

By the way, you will still need to maintain the deck after staining and sealing. The stain will last for quite a while, but you'll need to re-apply the water seal at least once a year, once every 6 months if you get a lot of rain.