SWIM has found that what works really well is saying something like, I have got paint and oil on my driveway, how do I get it off, then listen to their crap for a while then buy the HCL, if they recommended it they have no doubts in their minds that thats what you are using it for. Also when you go back next time, tell them it worked great, let them show off their knowledge, let them suggest it, and then thank them....
Worked for SWIM
Peace
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Buy the ticket, take the ride.