We use rapid tac on back lit images, due to the fact we have had issues with homemade application fluid causing the vinyl adhesive to have splotchy white spots.
Everything else we use 4 pumps (I think, I don't mix it anymore) from a off brand of baby soap with a pump (it says baby wash, but has the same ingredients) ,and tap water, in a 1 1/2 quart hand pump sprayer, like this: