Most printers overlook this important step and end up with premature breakdown.
When you initially expose your screen you’re just trying to get the image on there without over or under exposing. The emulsion is basically soft and pliable until this last step (which is known as post hardening). Post hardening is done to finish exposing ANY emulsion and solidifying the screen for production. You wait for the screen to dry for a couple of reasons; as the emulsion dries it shrinks, the moisture is removed and the emulsion increases in density. If you try to post harden while the screen is wet they actually work against each other; the screen is hardening and solidifying while the moisture is still there. This will lead to pinholes. Using emulsion to block out will initially allow for a stronger bond to the screen since it’s the same material and it can be hardened with the rest of the screen. Usually 10 minutes in your exposure unit will do (five minutes in the noon day sun is better) After this you have a screen as good as the professionals make. If you happen to get a small pinhole during production use a commercial block out as they are designed to dry quicker for on press.