hi there
firstly, retention and CIS are totally different things, dont get confused.
secondly dont agree to retnetion on any clenaing activities, the job is done, and thererfore IT IS CLEAN, get the work signed off as soon as you have done it sp graffitti, photograpgh before and after, and then get paid,
if you dont , then the kids will be back with there spray cans, and thats your retention gone.
good luck
regards
martin