I stopped using enamels five years ago, due to the fact that they dont last as long anymore, or chalk easily. I do alot of signs made from mdo, and I use a high quality latex based paint for all my backgrounds. I always use satin finish and have never had a problem with adhesion or lifting. I used Porter for awhile, but now use Kilz brand which has a twenty year life. WHen I first started using latex, there were some issues with high gloss. It seemed to emit an oil after drying at times. After I went to semi gloss and then to satin, never had that problem anymore, and, the better vinyls stick much better also. I use Oracal 751 on all outdoor signs and it sticks great.