I'm a professional mechanic. It's incredible that someone who actually looked at the car for a metal grinding noise would not have seen what you saw. If they didn't look, they're idiots--dangerous idiots at that. Unfortunately, there's much more demand for auto repair than there are qualified and competent people to meet it, so the partly competent fill the void, sometimes with sad results such as you describe.

In my nearly thirty years in this field, this is the first time I've ever heard the notion of powerwashing brakes. It strikes me as ridiculous.

The wear indicators jeffp refers to make a screeching or squealing noise when the pad linings are about 90% worn. It's a different noise from the grinding sound that is made when the pad wears completely and its steel backing start chewing up the rotors as wdyat 12 described. Some original equipment pads have these indicators, some don't. Some aftermarket pads have them, some don't. The better brands of aftermarket pads will have them in applications where the original equipment ones did.

Click and Clack pretty much know what they're talking about, but the constraints of the radio show format sometimes limit how helpful their advice can be. In a situation where the problem is caused by "A" 70% of the time, by "B" 20% of the time, and by "C" 10% of the time, they're going to say it's "A". If they got into all the technical details, they'd lose the show's momentum and bore most listeners. The show is much more an entertainment device than a rigorous technical resource. Still, within these limitations it can be a good source of information.

I may be able to help with finding a good repair shop in most areas of the U.S., and some overseas locales as well, through a particular internet network of automotive professionals. If anyone is interested, send me a Mudcat Personal Message, and I'll see what I can do.