Bicycle Mechanics - Seat Tube Slightly Too Big for FD Clamp

I'm building up a new custom frame with a 28.6mm seat tube. My 28.6mm front derailleur clamp is slightly too small to fit on the tube. I measured the seat tube diameter with a piece of paper and a calculator, and it is 29.5mm. I assume this is due to the double layer paint on the frame. The 1mm difference is quite large and I'm not going to risk crushing the frame. Has anyone encountered this issue before and what is the best solution? Should I file the FD clamp or get the next size up and shim down? Thanks.

Edit: The seat tube is True Temper OX Platinum with a wall thickness of 0.4mm. I filed the clamp. It took over an hour and installed fine; the amount of material I had to remove was significant. No way I was risking a $3200 frame for a $15 front derailleur. No, I did not just file a 28.6mm clamp to fit on a 31.8mm seat tube. It's a custom frame and I spec'd the tubing myself. The seat tube is 28.6mm. To Xenologer below: the FD came off of a Surly Cross Check that got run over by a car. The Surly was a 2009 and the FD fit it's seat tube with no strain whatsoever. Perhaps your year had thicker paint.

Technically, you've only calculated the seatpost diameter, not measured it.
Only actual measurement is the circumference.
Check again with calipers to be sure. It sounds way off.


I think the only thing you'll crush is some paint, not the frame itself. (I'm assuming this is a steel frame)
Have had thick paint on some Surly frames, just went with the specs listed and squeezed the clamp on. No probs. Had to swap clamp bolts to get threads to engage is all.

If you still don't want to do that: I'd rather shim down than start filing at the derailer.