White out does work (and white out tape, which is much more fun to use), so does white acrylic paint (make sure it's not a transparent color of white--the bottle will usually tell you how transparent it is) and I've even read about people who just straight up cut the top layer of the paper out with an exact-o knife (though I feel like that would take longer? You'd have to have thick watercolor paper)
But, when you go to scan, you will still see a little bit of the edge just from the shadow (that and the paint might be lighter than your paper) so you'll still have to go into levels and take off the top 10-20% of the light values and probably bounce up the darkest values, which you should be able to do from your scanning settings if you don't have Photoshop, CSP, or Gimp. It's best to go into levels anyway, to get rid of any old pencil lines and texture from the paper.