WFP does a perfectly good job of sills, and if you already do a good job of cleaning your customers sills with trad methods, the sills will already be clean to begin with, so keeping them that way isn't a problem.
On new cleans with grubby sills then you do have to put a bit off effort in to get them to come up well, and the higher up you go the harder it is (for obvious reasons)
Once you have got them clean, keeping them that way is no bother at all.
Ian