Serpentine belt driving me nuts
 

My OEM belt started squealing at 17k miles so I got a new one and that one later started squealing too a few months in so I put aerospace 303 on it and it
quieted it for about 8-9k miles. So at 26k now it started back in so I changed it again but it is squealing immediately.
The dealer says there's something on the belt but on this new one I degreased all the pulleys until they were 100% grease free and put the belt on.
There is nothing wrong with any bearings, tested them all with the belt off. Dealer said to clean the belt with brake cleaner and that made it squeal worse.
He said it could be the tensioner but it's tight and the belt just barely gets on when installing it so it doesn't seem like it's that.

So I gave in again and sprayed Aerospace 303 on it and it's back to quiet.
Any ideas on what is going on here?

Well, I checked with Bubba and he says to apply some belt dressing to the belt.

Also, iffen you don't have any belt dressing on hand, you can use the sap that oozes out of a cherry tree, just grab a ball of it and hold it against the belt as it rotates.

As a bonus, if you find a cured oozing ball, you can chew it like bubble gum.

You're welcome.

It's my nap time. I hope ya'll can get along without me for a couple of hours.

Check pulleys for alignment, also, check the alternator pulley to see if the bottom of is grooves is polished.

Seen quite a few alt pulleys worn down with the peaks of the belt squeaking on the bottom of the pulleys grooves. The belt should only touch on the sides of the Vs, never on the tips

Stop spraying protectant on it.
Check pullies again as RZN said.
If still noisy get some proper belt dressing as Hum said but that is a last ditch option if you can't find root cause.

Check inside the grooves of the belt for hard contaminants like grains of sand. I saved my belt by carefully picking out the shit jammed into it. Was amazed to see how much it had collected. This is now part of my regular maintenance.