D.N.
I have done the bathtub method. But, if you can hang them outside, it is much easier. I place them on the clothesline in the yard with the weight of the top part holding it on and hose them down. Usually the dust comes right off. Any bad spots are easy to wipe. You can use a solution of vinegar and soap if they seem really bad. I put this is a spray bottle and give it a minute or two. I try to do it this way when it first gets nice out and then again before the weather gets bad. My husband sometimes vacuums them in the winter to keep the dust down.