The Scrubby Brush
This has to be one of the best kept secrets in painting: The Scrubby Brush! Let me begin by explaining one of the most frequently asked questions I get is something along the lines of, "I try to paint the runes etched in my mini but it's impossible to do without messing up my beautifully blended armor! Help!" The Scrubby Brush (seen below) is going to be your new best friend in painting.
I always make sure to have this brush on hand no matter what I'm doing. Whether it's painting glowing runes, freehand or just getting a mark on a surface that has already been painted to completion, this brush comes in very handy.
The Scrubby Brush is just an old nylon brush you may have hanging around that's been cut short with scissors. You must use a nylon brush for this because the synthetic bristles are stiff enough while still being gentle on your paint. A sable cut short won't be stiff enough to scrub the paint off. First, find an old synthetic brush in your bin and some scissors. Then cut the bristles short.
Using a mini that I've just base-coated, I'm applying a dot of blue paint as my errant mark.
Now, you need to move quickly to gently scrub off the errant mark. Dip The Scrubby Brush in your rinse water and apply the water over the errant mark. Let sit for a few seconds then start gently scrubbing the mark using a circular motion.
The mark should rub off without much damage! Sometimes your undercoat will rub off a little bit but if you are gentle you should be able to repair the damage without much issue.