Primer bottom of roof best way.

You can get collapsible bladder type cups. These collapse with vacuum allowing a gravity feed to spray in any gun position.

X2! Best in my opinion. I just bought new primer guns from the paint store. They are DeVilbiss, they came with the liner setup and 10 liners for $85. That was 2 guns!

If you don't want to spend money just go to the local Walmart and by a roller and roll it on. No one will see it anyway! Less mess!:D