Paint a deck when wet weather is forecasted

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.

If the tub material is fiberglass (and most commonly are) the best way to remove any blemish is with an orbital sander and some 300 600 and 1000 grit Wet and Dry sandpaper. Using a spray bottle filled with water wet the stained surface. Starting with the lowest grit sand and keep the area wet. Let the sander do the work. No need for hand pressure. Watch how fast the stain is being removed (it should go quickly. Move through the grits and spray to keep surface continuously wet. The paper is of such a fine grit it will not leave any discernible scratches. When completed apply a cleaner/sealer for fiberglass.