I was talking to a house-painter friend of mine the other day and came away with two bits of advice. The first was about the color of the paint and the second was about brushing thin finishes on vertical surfaces.
As most everyone knows, lighter surfaces reflect light better than darker surfaces. So the warning from my friend was to avoid using a dark paint in sunlight on a hot day. The paint can get so hot that it will blister. Instead, apply the paint in shade.
Painting vertical surfaces is not usually a problem, because most paints are water-based and are made thicker, so they don’t run very easily. But applying clear finishes can be a problem. They tend to run and sag on vertical surfaces. So the trick is to brush back over the areas where the finish runs to pick up some of the excess with the brush’s bristles. Then spread this excess on a bare area or wipe it off onto a clean cloth you hold in your other hand.
You may need to do this several times to remove enough finish so it doesn’t run anymore.
Always remember that you can’t see what is happening on the brushed surface unless you look at it in a reflected light. You may need to arrange some lights, and you will probably have to move your head to see a good reflection.