I highly recommend a gun. Even the $25 spray guns at harbor freight do a really good job. Take your paint and add water to it until it's a thinner consistency. Then spray with the gun and you will have a MUCH better outcome. Trust me!
I doubt RMarshall would have been happy with results of that recommendation. I'm guessing the Cover Stain was the oil-based variant and most urethane is also oil-based. Water where you should be using mineral spirits is never good. And just so you know, most water-based paint recommends maximum of 10% reduction. Some say not to reduce at all.
