A signpainter will use "Artist's One Shot"
Back in the Dark Ages people would paint signs for a living ... with an actual brush. Their universal choice was (is) Artist's One Shot. It covers in one application, hence the "One Shot" and is "flowable" meaning it comes off the brush cleanly and evenly. It was formulated for this purpose and comes in a selection of colors. It cleans and thins with turpentine or mineral spirits. Ask for it at an art supply store or if you can find one, a sign shop. More important than the paint is the brush. Ask at the art store for a quality lettering brush and a quick, 2 minute lesson to make sure you're getting the right size. Then give it to your friend afterwards for thanks.While you are in the store ask about real "Gold Leaf" as well, but prop your feet for the cost. Chances are your artist hasn't had achance to work in that medium since art school and may like the challenge when it's on your nickel.Ol'Dave