Brake pad thump at the rim seam
 

While braking the brake pads thump on every rotation when it meets the rim seam. I’ve noticed this on many rims, even when the seam joint looks pretty clean. Are there any tricks for reducing this tendency? Thanks.

I got a small nick in my front rim, first I used am emery board between the rim and the brake pad, gentle pressure on the brake and rotated that spot on the rim back and forth to sand it down, then used some 150 grit sand paper on a sanding block to further smooth it out. Be gentle, don’t do more than you need to make the sound acceptable.

In increasing order of effectiveness:

Brake a lot and wear the rim out. Replace the rim.

If you build your own wheels, turn the rim around in the wheel. Or reverse the skewer (front wheel only).

Emery board, file down the joint. Problem is, there's often a broken pin in the rim, and the joint only "opens" when the brake is applied.

A few swipes with a flat file have resolved those issues for me. A file is easier to control than hand-held sandpaper so you will better target the high spots.

IME:
Try all manner of sanding and filing and straightening, with little effect.
Tolerate until annoyance level reaches peak.
Get new rim/wheel.

Personally I'd try to understand which of three things is happening:

- mismatch in the position or shape of the joined ends - seems like you ruled this out.

- rim not true at the joint to a degree that spoke tension can't fix - I have a wheel that has this, fortunately there's enough float in the caliper that it hasn't proven an issue (but then my brake is only for drag on moderate hills, not stopping)

- rim profile distorted on both sides of the joint such that the ends much each other but not the rest of the wheel - you might try using a pair of calipers or better yet a micrometer (for the flat faces more than precision) to measure the width there vs. elsewhere

Any attempt at improvement beyond removing material is probably only something that can happen with the wheel fully de-tensioned, while built up that joint is under immense compression.

If it's a front wheel, turn the wheel around so the lip passes on the trailing edge of the seam. Sometimes that's enough. Otherwise, or if it's a rear wheel, emery board, flat file, etc. as mentioned above.