Controlling gel coverage
Let me preface this by saying that I have zero experience with Hammer and have only created maps using the puzzle editor, so the following may well be entirely stupid.
I have a map which involves dropping repulsion gel onto a flip panel, and washing it off. There is a slight problem whereby a player who messes about with the droppers enough may succeed in getting the panel only partially covered in gel, and thus bypass the intended puzzle. My question is, is there a way of using Hammer to ensure the panel can only be either entirely covered in paint or not at all, by modifying some property of the paint dropper, the func_door_rotating object, or the level itself?
Sorry if this is something that's addressed in the various existing tutorials, but a preliminary investigation has shown me that I'm not exactly sure what I'm looking for. In my head I picture some alteration of the 'resolution' of the paint coverage. I would not be surprised to learn that this is not a thing that can be done, though. If anyone has any input or suggestion, it'd be most appreciated!
Would it be okay for your puzzle if you used paint bombs instead of a fluid stream of gel? I think you can change this inside PeTI.
A map pack coming soon. - click for more information
It's possible to control where you don't want gel to paint; this way you can force the player to paint exactly the portion of panel you want, but it's difficult (but possible using a really complex workaround) to force the player to paint a whole surface if he can make use of some bits to bounce and continue with his unintended solution.
You can avoid that the player paints a surface or by turning it into glass, or adding an entity called trigger_paint_cleanser
useful tools and stuff here on TWP 
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Hello ! They are some specific white and black panel textures in hammer that can't be covered by gels. It don't exactly remember their names.
Unfortunately because of the way the room is set out, when I change it to a bomb the gel dispenser loses all sense of aim and can't hit the tile it's aiming at properly. (In fact, it will hit the ceiling unless I extend the room upwards!) It might be workable if I restructured the map... however, maybe I can alter the trajectory of the bomb in Hammer? Is there an idiot-proof guide for this somewhere?
Edit: something to do with the launch direction on the dropper's trigger_catapult, I presume. I'll see if tinkering with that helps at all.
Further edit: that didn't help in the slightest, but setting Use Exact Velocity to "Yes" did the trick. The bombs were still overshooting slightly so I moved the target a bit as well. Problem solved! The map is here if anyone wants to play it.