Sales is a tricky thing to do correctly. First off, you are the expert, not him. You have to tell him what he needs not ask for his opion on something like this. You lead the sale. When he gives you a checkup you don't tell him how to do it. You HAVE to be confident and show him you are. Next explain to him the science behind the process. Why you have to do the things you do and how they are accompilshed and effect they deliver. One other thing is even though you are using A PC Orbitable don't tell him not to worry because I'm not using a circular. Its sounds to him like you are not comfortable using one, and this instills even less confidence in his eyes. Instaed of saying " Don't worry, I won't use a machine that I could damage your finish with"(To him a machine is a machine), Try saying"For this application, I WILL be using this technique to do the job properly. "Be confident" Don't let him lead you!
Sorry if this sounds confusing it's hard to wrap up proper sales techniques in a manner which is quick to write and to read. Be very polite as you are trying to service your customer and make him happy, but in order to do your job properly you have to do it the correct way.
Like the other guys said, show him with picture or a demo on site. Even if you already sold him with confidence this is a great way to reinforce it.
Remeber "YOU KNOW BEST" your the pro. If you let customers tell you how to do it, you are in for a world of trouble.
I am not saying to be mean and loud, just let them know that in order to do the job properly this is what I have to do.
Worse case sonario, let him walk. If you do the job his way and the results are bad, guess who looks bad and never gets a referal.
"A Happy customer tells a few. An UN-Happy customer tells alot"
There are a million sale techniques I could give you. If you want any more please PM me.
..................Clint