To fix a smudged nail as you're applying the polish to your fingers, don't panic. Take your finger and just gently wet it with some nail polish remover. Try to smooth out the nail polish. Hopefully, it isn't that bad. You can just re-wet it a little bit, and you can see it actually smooths itself out. Wipe off your finger of course. Take the same nail polish and fill in where you've removed a little bit of polish. Not too much.
Continue on to the other hand if you still have to polish the other hand. If not, let it sit for a couple of seconds and dry. Then, you can either go over it again, the entire nail with polish, but I recommend just following through with a top coat. Because the top coat will fill in the line if there is a line or a difference in the way the nail looks based on where the smudge is. And I'll show you.
So, I filled in the smudge. I'm going to apply a top coat now. I'm going to start right in the corner where the smudge was. You'll see it literally smooths it right out. That's why it's also great to use a good top coat, because you want it to hide imperfections especially when you're doing a manicure in a rush. It's almost like a magic worker.
Another tip is if you're rushing and you realize that you've left polish on the skin, once you're applying the top coat it re-wets the nail polish a little bit. And it gives you an opportunity to clean it up without messing up the nail.
So, there you have it. A really quick way to correct a smudged nail without having to do your whole manicure all over again.