On a flat surface any full surface weight will work to some extent. I'm trying to prevent any shifting of the latex and to spread the filler or glaze out far enough so that there is little or no edge.
This really became an issue with the curved edges at the bottom of the door.
The inward curves were really tough. It's hard to keep the latex in contact with the filler while it dried.
That's where vacuum comes in handy -- to have the pressure differential push the latex into the curve and especially where the weatherstrip is glued.
Plus, the pressure applied to the surface is hard to beat without a vacuum.
12–25 inches of mercury equals 6–12.5 psi on the latex. This pressure is almost impossible to duplicate without "clamping".
http://www.westsystem.com/wp-content/uploads/VacuumBag-7th-Ed.pdf Section 1.2
I should add that another benefit is one that Jim mentioned. Vacuum will draw out entrapped air. This works well for removing air bubbles from urethane (and other media) poured into a mold -- and from glaze.
After a week of trying I've found this to be very difficult on curved surfaces. After half a dozen tries I have one surface that is tolerable for the bottom of the door. The vacuum system does not work as well as I had hoped as it leaves a wavy surface.
A brayer (roller) is working better.
More updates later.