I have a few questions that someone can hopefully address concerning painting two off-white wall areas (approx. 10' x 12') used as screens in a church setting in medium ambient light using two Dukane 2200 lumen LCD projectors. From what I've read, the most economical route to go concerning painting walls to be used as screens is to use the Behr Silver Screen paint. Some have said it is best to mix it with flat Behr UPW 50:50. Another has said mix it with silver metallic 75:25. Yet others have suggested other mixing components. Considering my situation as described above, what economical paint mixture would be the best? Is a primer needed on an off-white wall. Is a top coat needed? Any other words of wisdom?