Spray bottles vary in quality. If you can find a janitorial grade of spray bottle at a hardware store or big box store, chances are good that it will be a better sprayer than something you might find at a dollar store or other cheap discount store. It might cost a little more, but it's worth it as far as I'm concerned.
Elmer's Glue-All and Mod Podge (acrylic medium) look a lot alike and act a lot alike, but have two very different and useful properties. Elmer's glue is not water proof when dry. After it dries, it can be easily softened up by wetting it down again with water. I like to secure ground foam and track ballast with Elmer's. Later on, if you want to reclaim ballasted track or make changes to scenery, simply wet everything down to aid in removing it.
On the other hand, Mod Podge, although it's water based, is very water-resistant when dry. It doesn't soften up easily at all if you re-wet it. I like using Mod Podge for tree building and modeling water surfaces (use gloss medium for shiny water surfaces, NOT matte medium). Once trees are foliaged and surfaces are watered, I don't have to worry about covering them later on if I come back in and do some more scenery work. If they get wet from spraying more water in the vicinity, no big deal. The ground foam stays put on the trees, and the modeled water surfaces don't turn into a gooey mess.
For either Elmer's or Mod Podge, you have to mix them quite thin in order to spray them. Too thick, and the solids don't want to atomize properly, thereby "splattering" out of the nozzle instead of "spraying" or "misting". I have found about a 5:1 ratio is about as heavy as you want to go with your scenery mixture (5 parts water, 1 part glue) in order to get a good misting "spray" out of the sprayer nozzle. Once again, too thick of a mixture, and it will start splattering out as opposed to misting out.
And adding a few drops of dish detergent to the mixture as a wetting agent doesn't seem to hurt anything either.
