Gel spreading
Does anyone remember Gelocity? I am creating a coop level which uses the same technique. However, I have no idea how the propulsion gel was spread across the entire level like that. I was wondering if anyone could tell me how to accomplish that. I know I can decompile the level and find out, but I would hate to destroy the experience of the level by looking "behind the curtain". If anyone can help, it would be appreciated.
Just have well placed info_paint_sprayers; activate them on mapspawn and de-activate them a second later 
I thought that was the way to do it. I just wasn't sure that would work. Thanks for the advice.
Best way of figuring that out is by testing 
I suggest creating a test map: Just a big empty box with an info_player_start and a portalgun. Adjust the map to your needs so you can test all different kinds of things in it! You can just use one (even misaligned) texture and one light entity in the middle of the room and you'll be ready to test. A map like this compiles very very fast and allows you to quickly check concepts for your maps!
True. I often build test maps for a concept I have. Hell, I have an entire folder of test maps. Hehe