Full Coverage coat of USG SheetRock First Coat Primer
 

I am applying USG First Coat Primer to my drywall and reading the directions I am wondering what is meant by a "Full Coverage Coat". I am applying by using Brush and Roller method.

In the video on their site for level 5 finish, they mention while explaining spray application method, that it should be 18 to 20 millimeters thick. Is this what I need for brush/roller application also?

level 5 is a skim coat over what's usually a nice level 4 finish. i haven't used the product you speak of but to perfect a level 4 finish you can roll on watered down compound or possibly use a primer builder. haven't tried the builder and it wouldn't qualify as an actual level 5 but it may give you the same look if the level 4 was finished nicely. plain pva primer is only made to prevent paint soak in, more of a primer sealer so to speak. if you paint new rock expect to use 3 coats. with pva it's 2 coats. since it's often 1/3rd the price of paint it's a no brainer on why it's used.