I collect and fix bikes for a charity. I have a 26" bike that has the brakes squealing on the front and I can't get rid of it. The wheel is true, I have replaced the pads. The pads are adjusted properly to the wheel and have a slight toe-in. The noise occurs all the way around the wheel. These are v-brakes and the rims are painted aluminum. Any ideas of something else to try would be appreciated.

I might try more toe in or even toe out.

Apply garden dirt to the new pads. Andy (don't roll your eyes, it has worked for me a number of times)

Yup. it works. I seek out the good stuff. Lots of nutrients, compost, etc, little sand and clay. (Just to spare the pads and rims some abrasion but even lesser stuff works for the ears. I've even used roadside dirt in emergencies.)

Now there was a time when I used to adjust those famous squealers (Mafac Racers) for the right amount and decibel level. My Boston days when I didn't own a car. Ben's Revenge. When a car pulled a nasty on me, I'd hit the front brake extra hard. That squeal could be heard blocks away! Every head turns. I sit up and point to the offending car. Every set of eyes follows my point. Driver is arisen from his stupor by the squeal and looks up. Guess what? Everyone in all directions is looking - at him! Driver slinks off like a bad cat that's been caught in the act.

That squeal made riding Boston's free-for-all streets much more fun! And yes, sometimes I had to apologize to innocent bystanders when I did actually have to stop and it wasn't an emergency.

Consider this post "the dirt" on brake squeal from someone who has had those famous squealers on a working bike or two continuously for almost 60 years. (With KoolStop pads, proper toe-in and not too much wear on the pivot bushings, now rarely an issue but those old Mafac pads were fun! And like the KoolStops, they were very good stoppers while the rubber was current.)

how much of the rims are "painted"? if the paint is on the brake tracks, it may be the problem...

brake squeal is a series of grabs/releases of the pads on the surface, which sets up a rapid oscillation we hear as a squeal... gloss paint is very smooth and very grabby.

and a good handful of dirt (mentioned above) can cure the grabbies.. i usually apply dust as the wheel is spun with a slight brake drag... it slightly roughens the brake surface of the rims...
light work with emery (Black) sandpaper can do the same thing... 120 grit works for me, on aluminum rims... regular (tan) sandpaper can leave glue on the brake tracks, causing more trouble.

The bushings in the brake arms may be too loose on the pivot studs. Tightening the bolt won't affect this. I had to replace the brake arms to eliminate the squeal. On my bike, if the front wheel is turned too far, the top of the brake arm hits the downtube; I suspect that this is how the bushings got loosened up over the years.

I would recommend if you are so inclined use some sandpaper to roughen up the paint on the rims (which will cause braking issues on its own).

I am going to roll my eyes a little Andrew R Stewart but in the end I believe it probably will work quite well, though would clean up with alcohol afterwards.

Rims painted or anodized? Perhaps try cleaning them with solvent/alcohol/detergent? Agree on roughening the pads with fine sandpaper, perhaps they're glazed or somesuch.

These challenges can be exasperating, good luck!

Clean the rim and blocks with acetone. Toe-in further, so the blocks are only flat on the rim with maximum lever pressure. Apply grease to the brake arm pivots. Roughen the brake blocks with coarse abrasive. Roughen the rim with coarse abrasive - you can put a piece of emery cloth on each brake block and ride the bike dragging the brakes slightly, or you can get serious with a grinder like some trials riders do.

In addition to everything already mentioned, check that the V arms are bolted on securely and don't have any play and are lubed properly. Sloppy/worn out arms can vibrate and cause pad squeaking during braking.

I discovered the hard way that painted-sidewall rims are horrible for inducing brake squeal. I ended up removing the paint (which wasn't easy), but that solved the squeal issue. I wrote it up at What have you been wrenching on lately?

i've used my 2" sanding discs and then placed the wheel in the Park truing stand to remove the paint... with a bit of trial/error, i found the angle to both sand away the paint, and make the wheel spin at a slow, even rate... an even, bare aluminum finish, without hassle, was achieved
IIRC, 60, then 120 grit discs.were used... been a while since i've spun one.
i finished the job with plain 220 emery paper, and a few spins by hand.
i considered trying 400 paper too, but it looked fine, so, meh..
The "paint" was powder coat... tough stuff!

pneumatic 1/4" angle die grinder, rubberized screw-on disc arbor, 2" screw on discs...

Amazing how experiences differ with each user. The painted rims on one of my touring bikes have never made any noise, however the non-painted rims on the other touring bike squeal nice and loud. Both have stock pads that came with the calipers when manufactured. The difference is the calipers themselves. The painted rims are mated to Suntour Roller Cam brakes. Nice heavy arms, brass bushings that fit perfectly to the studs. The other non-painted rims are mated to Suntour XC Pro cantilevers, which were notorious for squeal because of the way they mount. Brass bushing as well, but the adjustment/mounting hardware makes them prone to squeal.

My suggestion is replace the calipers with higher quality calipers. Avoid the old Shimano XT articulated V-brakes as they like to squeal.

For bike brakes, I like me odorless mineral spirits. It doesn't attack paint or plastics, and it dissolves practically all oils, waxes, and greases. Clean the rims and the pads, and you're good to go.

However (added in edit):

Don't soak decals in solvent. Be careful around them. Odorless mineral spirit tends to dissolve decal adhesive if left on long enough.
Denatured alcohol can wreck the surface of decals. I have had bad luck with silvery decals getting ruined by alcohol.
Otherwise, odorless mineral spirits and denatured alcohol are pretty safe ways to clean plastics and painted surfaces.
Acetone and ethyl acetate (nail polish remover) will destroy decals and attack some paints and plastics.
Methylene chloride and tetrahydrofuran; first, why do you have that stuff in your garage? These will go after decals, paint, plastic, and powder coating. They cause cancer and reproductive harm. They can also destroysrubber brake pads and some tires.

When in doubt, test it on a small inconspicuous area. The day it destroys something that the world will never see is the day you will be glad you did.

I have a AC rotary tool and some little abrasive flap discs that should do the job, although it would probably wear out a few of them - I've not had to strip a rim since I got the tool (I always considered Dremels to be toys for people who don't have die grinders or angle grinders, but I thought I'd give it a go anyway).

If my back brake squeals I leave it alone - I mostly just use the front so it's not annoying, but a light touch of the lever and a squeal grabs the attention of errant pedestrians. Bad drivers mostly get shouted at.

Very helpful thread. I had a squeaky front brake that persisted even when I changed wheelsets. Made me think it was a pad issue but cleaning did not solve the problem. Seeing this post made me check the caliper arm tension and indeed, they were ever so slightly loose. Made the adjustment and voilą, no more squeak. Thank you! I am always learning when I read this forum!