Time is money - charge your shop rate for the cleaning - you could be doing something else that is profitable. Be up front with your customer as to the charge and let them decide if they want the service or prefer to do it themselves. These types of people many times will want to take advantage of your good nature. I got in one of these types of orders last week and the plaques they wanted refurbished were just when they were brand new - I check the name on the back then did a search to see if they were still in business - not surprisingly - they were no longer in business.
Wonder why?

I do not mind helping someone out and saving money for them but I also refuse to put out something that is less than a quality product. Redo sometimes cost more than what a new quality one would. I let the customer make the call.