There's multiple ways to cleanly apply wash, but all of them require a few extra steps or effort:

• Use a very thin and sharp brush and only put the wash in the areas/cracks you want them to go in. Very time consuming, but is one of the neatest methods. You can correct any spills or mistakes by quickly wiping it away with water or waiting until its dry and painting over it.

• Slap wash all over the model like your life depends on it, then enjoy the darker model or go back when it's dry and repaint all the surface areas with the original paint color. This method works very well for metallic colors, you can just put wash all over it to get a nice look.

• Spray the entire model with gloss varnish, the glossier the better. Then slap wash all over it like the previous method. The gloss will make sure the wash naturally flows into the cracks on its own and is easy to clean/won't be stained by the wash as much. When it's dry, spray matte or satin varnish to remove the glossiness.