we are preparing our home in NH to sell where mildew is a frequent problem.  Our wood siding is 27 yrs.old and in pretty good shape, but after the last staining about 5 yrs. ago it developed mildew for the first time.  We want to freshen the exterior to sell and don’t want to cheat by staining over the mildew but don’t want to make a huge investment either.  What is the cheapest and easiest way to prepare the siding for stain? (Cabot, barn red)