you dont need a weed killer the pressure from the jet washer is enough to shift most weeds
you can repaint the monobloc with a clear stabiliser this suppresses weeds
as it acts as a sealer, also prolongs the length of dirt build up, but then this is another service and most people wont go to that cost, however if they ask you can offer this service
you brush the stuff on with a roller does not make the monobloc look any different as it is clear, you can view the stuff in bnq to get an idea what it is about then look about for cheaper ulternatives
but as I said most people will not run the etc cost and you are doing yourself out of a job for next year as they expect it to last a few years
in my views better not offering
jinky