I know the sills you mean, you should probably be looking at a suitable primer rather than a power sander…(a decent primer and topcoat should last 5-10 years rather than 2)
Fence, similar, I can’t remember ever having to sand a fence (With the exception of picket fencing) a good wash down….creosote isn’t available anymore, so you are are probably using creocoat…..a completely useless product.
Try “Ducksback” or similar.
Properly applied should again last at least 5 years.
Any way, pics of your exact problem might help……..