First thin the Clear Shield. You can thin it up to 20% per the Clear Star rep I talked to. I thin it until it's about the consistency of milk, whatever percentage that might be.
Then lose the roller. You want to mop it on with a foam brush or an actual mop. You want to be just on the wet side of a dry brush. Blob some on the vinyl and spread it with overlapping vertical strokes. Then do overlapping horizontal strokes. Or vice versa. Whatever you do, keep the coat thin. Adding more coats or a thicker coat serves no functional purpose. Once the media is coated, it's coated. Further application is just coating the Clear shield with more unneeded Clear Shield.
I know from whence I speak. I've been using the stuf for years and once I realized that thinner is better, I've never, as in ever, had a problem with it.