If you are thinking of "skiming" the whole house or even a handfull of clapboards...over multiple layers of old paint??... you are asking for trouble...
There would be bonding, and flexibilty issues...
Exteriors are not designed to be "skimmed" over... especially clapbords...
Old houses are what they are... if they have to many coats of paint on them... it calls for removal of the old paint... there is a such thing as a limit to how many coats a house can handle...
If the home owners are looking for a "smooth" surface.. its either a strip or a replace clapboards...
I dont beleive there is anything on the market that you could use on an Exterior to "skim" over multiple layers of old (scabby) paint and there not be a problem...
Of course there are "small" areas and times where you can skim over something...Like a dent on the trim or something...or a door etc... Im not saying there is not ANY area...
There are options....
www.silentpaintremover.com