Yeah reading again you are right. My experience is with the paint, not the prep. I probably would still brush on the prep though since overspray on plastic, glass etc will probably cause damage from the acid.
I tried spraying it once and it didn't adhere very well. Think it did not care to be thinned. Brushed has been better for me too, no issues doing it that way and it smooths out enough you can't tell it was brushed on.