Don't know for sure what kind of texture you're dealing with but it sounds like what you're looking for is skim coating. There are sprayable high-build primers but that's more for a level-5 drywall finish of new walls, not things like getting rid of a knock-down texture in an old house.
Check out this youTube video. This guy has a lot of useful tutorials, more than one of them covering skim coating, and generally provides lots of good information in a clear (and occasionally entertaining) manner.
In a nutshell, for skim-coating, you thin joint compound to a soupy relatively thin consistency, apply it with a trowel or paint roller then wipe it off with a wide joint knife (or trowel (or special skim coating squeegee)). I won't take on describing how much to thin it down, the video does a much better job than I can do verbally. It takes some practice but goes pretty quickly and you can work with a 2nd person: one rolls the thinned mud on the wall and the other follows behind skimming it off.
A few tips from experience:
- I would recommend using drying compound. If you use setting compound, it may dry before it sets leaving it powdery and crumbling away.
- If there are already many layers of stuff on the wall (plaster, different types of paint, etc) you may end up in a situation where the skim coat sticks but applying paint makes it lose grip and bubbles up the paint or leaves you with pinholes. I understand this is because the moisture is essentially trapped in the skim layer. If that happens, you'll need a sealing primer.