I forget which car it was on, but, I believe my camaro I had the squealing after I had someone else do my brakes. I had brought it in for Tire replacement and they pointed out the low pads. Normally I'd say no, but, the price was right so I let them do it.
That was the start of my troubles.
Squeaked to hell, so I brought it back.
"Oh, that's normal, they all do that."
"Um, no they don't, the pads you just changed NEVER squealed"
"Oh, that's because these are a different compound"
"Well I want the ones that don't squeal"
"They don't last as long"
"I don't care, the ones on there lasted me long enough." (I don't want to hear squeaking for even LONGER than what the others lasted for!)
"Well, we don't sell those pads but we'll see if we can get it to quiet down."
So, they sanded the pads and the rotors and sure enough it stopped squeaking. For about 2-3days. Then it started up again. I brought it back once more and they sanded them 'better' but that it would likely happen again. I asked about 'anti-squeal' compound, "Oh we put that on all our pads so that's not it".
Sure enough, maybe a week later this time, bad squealing again.
I delt with it for a while before I ended up replacing the pads myself. When I took off theirs there was NO anti-squeal stuff on the backs. Put on new pads (metallic=hard=can squeal) and used anti-squeal stuff that Roger recommeded above. No more squeaks forever now.
MORAL: That stuff does work well. And never let a tire shop do a 'tackon repair' (for cheap or not) if you can do it yourself.