I think the problem is holding the can ! You have to spray evenly and go the whole length of the dash in one pass. Light coats are better than heavy. Make sure you shake the can well and keep shaking after each pass. Also, make sure to press the nozzle all the way down - I noticed that failing to hold it all the way down results in "dry" spots, almost like its spraying whatever the additive is but not enough of the liquid.

Here's a shot of two pieces - original versus the rustoleum