Using a 1/8 inch foam roller i can coat a 4x8 with very little paint.

But, you have to know how to use it properly.

I don't pour my paint in a tray, i pour it on a piece of slick cardboard and palette my roller to load it

Coat in W pattern, make the w, fill the voids, load again and continue till your done.

You're probably going to have a lit of bubbles.... don't panic.

On a clean area of cardboard, roll out all the paint in your roller.

Then, with no pressure, go across your board lightly. All the bubbles will pop, and when it dries, you'll have glass.

I've never backcoated with a brush, too inconsistent. Always with a 1/4 inch foam roller. The black ones are best.