With that said, here's the reality of that particular scenario. Painters do put water in the paint, but not for reasons you would think. Some materials need to have their viscosity manipulated in order to slow drying time, allowing gravity to 'smooth' out the product for a better finish. It also prevents 'drags' and 'sausages'. I personally try not to do it too often, but from time to time I have to. I want my client to have a proper finish.
The cost to paint metal siding -- most commonly aluminum and galvanized steel -- will depend on a variety of factors. In some instances, a simple cleaning will suffice to prepare metal siding for painting. In worst cases, painting professionals will need to repair metal that is rusted, damaged or missing pieces due to climate conditions. In the case of severe damage, a professional will have to remove the old finish and paint to get to the original layer, which will add on to the final price. When you add the actual painting of the metal siding, you might be looking at a final bill between $400 (~250 sq. ft.) and $1,500 (~1,000 sq. ft.).