The greatest art tool known to man.

I’m not going to apologize for this gross exaggeration later, and I think when you use this tool, you’ll see why. Actually, you’ll probably thank me.

If you’re a watercolorist, you know that one of the most challenging things about this medium is that it’s unforgiving. It’s just the nature of a transparent medium. Everything you do will be visible forever, and it’s a hard medium to control in the first place.

There is little you can do to really mitigate that reality, but there is a tool that can help you out:

This is the closest thing I have found that can bring your paper almost back to white after you’ve painted on it. It’s incredible.
To demo, here’s a little color swatch I did on cotton paper:

And here, with half of it magically erased:

As you can see, it’s not 100% back to white, but it’s pretty damn close. And with another layer of paint, you’d never know what was there before.

My method of using it is to cut off a small part of one of the larger erasers (about a 2″x2″ square) dip it in water and wring it out. If it’s sopping wet, you won’t have as much control over the erasing process.
With your damped eraser, cover the area in clean, smooth strokes. If you scrub, you’ll blast right through the paper. Think of it as wiping off the paint, and you’ll have a better time.
Once the paint is gone (or as gone as it’ll get), you’ll see a ton of little pills on it like an old sweater. No biggie, those are just the remnants of the layer of paper you just wiped off. Wait for your painting to dry, and then brush them off.
Voila! A new painting surface.

Now before you go erasing everything you’ve ever painted, there are limitations to this incredible tool.

You have one, maybe two rounds with this before it destroys the paper.
Watercolor paper is delicate. Some papers are more delicate than others (100% cotton handmade paper is pretty hardy), but they all break down after a certain amount of abuse. It isn’t canvas. One pass with this will remove most of the sizing on the paper, and the next layer of paint will not go on as evenly. But it’ll work. Two passes, and you’re done painting on that part of the paper. Three times with this tool, and the paper will likely start to rapidly disintegrate.
So use it wisely. It won’t save your painting too many times.

Nothing can bring paper back to 100% white.
Well, except carefully scraping the paper with a razor, which I do sometimes. But since that destroys the top layer of paper, scraping isn’t a viable option for removing large sections of a painting. The reality is that once you get paint on watercolor paper, it’s just not going back to white. This tool will get it pretty close sometimes, but never all the way. Keep that in mind. You still have to painstakingly plan your whites.

You’re still pretty screwed if it’s a dye color.
Remember the pigments vs. dyes episode? A dye color actually changes the color of the fiber in the paper. You can’t really scrub that out. If you use this tool on a dye color, you’re going to be scrubbing actual bits of paper away to remove the color. Which will work, but it’ll be much harder on the paper and you risk ruining it. So you still have to be careful with dyes.

Other than those caveats though, meet your new best friend. Go buy some and keep them handy. They’re truly the best.