Well, paint mist actually, that's where I've been residing in for a bit. I can't stress enough at this point that when painting, particularly in a confined space like my loft, you wear sufficient PPE. Latex or similar gloves, a face mask with appropriate filter cartidges (a dust mask won't do!), and try and vent the space a bit. (I used a desk fan to vent more fresh air into the area.). Paint fumes are not good for your health.
I've just been spray painting on clouds and mist etc on my backdrop, and I have to say I'm fairly pleased with the first try. I may touch it up here and there, but I don't think I'm going to attack it with a roller and fresh blue paint to start again. The key is work in moderation, mist the paint on, and keep the spray can (or whatever you choose to use) moving. Too long in one spot will result in a white dot and that means start over in many cases.. Misting on the clouds so that a bit of the blue back ground still shows through makes them look the most realistic. Be creative, I cut out only a few templates, but used them upside down, back to front, overlaying each other etc to create variety in my cloud shapes. Go over the top side of the clouds a few more passes than the bottom so that it looks like the sun is shining on top of them. I used mostly white, but I carefully used a few grey mists here and there (there's a bit of rain in the air). Grey was a much stronger colour though, so I toned that down a bit by carefully passing over with white again. Then finally, I misted the whole back drop from a larger distance, and misted the lower parts more than the top parts to make it all 'blend' together and create a better sense of distance. Now I have to wait until the mist settles down, before I take a picture and show the results to you. Time for a cup of tea first.