You should never have runs or sags in your dried finish. The trick to avoiding them is to spot them before the finish dries and remove them with a brush. The way to see runs and sags developing is to look at the surface in a reflected light, as shown in the accompanying picture. You may need to move your head, walk around, or even arrange some special lights other than the overhead lights or the light coming in through a window.
Once you have spotted the problem, use a brush to brush out the excess finish even if you’re spraying. If there is too much finish to brush out successfully, lift some off the surface with the brush and deposit it onto another part of the object, wipe the brush dry with a cloth, or drag the brush over the lip of a jar or can to remove the excess.
In most cases when repairing a finish (for example, removing a water ring, touching up a scratch or ding, or doing a burn-in or epoxy fill to a gouge) it’s most important to get the sheen the same as the surrounding surface and make the surface feel smooth. Even if the color of the repair doesn’t match exactly, the repair will be successful because people will think the mismatch is a flaw in the wood.
If you don’t get the sheen right, however, people will notice the repair immediately when they see it in a reflected light. Even if the do get the sheen right but not the color exact, most people will run their fingers over the repair, so it’s also important to make it feel smooth.