Doug,
There are many ways to protect these and other types of railing systems as well as other areas of the property. The quick and easy way to protect anything is to wet it down first. By wetting an area down combined the watered down product that may get on the surfaces it will be so diluted as to not harm the substrate in most cases. Now in instances where you need more protection so as not to cause damage you may want to first "cut in" your chemical. This term is used both when applying chemicals and also when sealing,staining, or painting. The term "cutting in" means you apply with a brush those sensitive areas where you do not want your chemical to be sprayed onto surfaces to cause damage. This is why you should also save your old brushes as this is when they come in handy. Then there are of course the use of tarps and plastic. I will give you two tricks I have learned along the way. The first is to buy plexiglass, it comes in 4x8 sheets and you can cut them down into 2x4 pieces and drill holes and put handles in them. These are heavy enough not to blow around and are very easy to keep clean. These shields have many uses in the field. The second idea is the use of the clamps that the kitchen exhaust guys use for securing the plastic to a hood. The use of these clamps with a tarp or plastic on a rail system work excellent! You can buy these clamps at any hardware store. I hope this helps you, remember that just because you use a spray applicator does not mean you do not have to do some work by hand to protect a customers property!