I use to oftenly work with blockout, directly on the screen,
this water based liquid is use to fill the hole on screens.
Since it's water based you can "erase" what you've made with water (Q-tips seems to be the best way to undo something), on the other hand you can only print with solvent based inks.
I oftenly use it to make harsh back layer of posters but i sometime work only with it.
To make it more simple
Here is a link to the full process.
Here is a link to the full bestiary i made with this technique.