Yes - removing the coating will remove some small amount off the dimension.
However, it may not be enough to matter. It really depends on how aggressively you apply the wire wheel.
So instead, figure out where your seatpost should be for your body size, and then mask off that point on the seat post and below. Clean and polish whatever you like above that line. Finally, blend the two areas together so there are no sharp lines or corners.
I'd be tempted to leave a centimetre or two above the frame/seatpost clamp and then blend to the polished part. You can always wrap a reflector or rear light strap around that point to hide it.
For the handlebar, same thing goes but leave the stem clamp area unmolested.
Another option might be to sheathe the seatpost inside an outer layer of purely decorative pipe/tube, such that it looks polished but the underlying seatpost is unchanged.
There are also chrome paints, or even nickel-plating to help change the "look" of a part without mechanically removing the old surface.
I have a spare piece of nominally 28.6mm seatpost tube in black anodised aluminium.
I read this as 28.50mm +- 0.1mm and will redo this with a caliper tomorrow. More info to come...
Update on Practical test
I decided to use the other side of the cut pipe from the images above. Total duration was around 30 minutes, based on timestamps of photos.
Initial diameter was measured three ways, with two dial calipers and a 1-2" micrometer.
Initial reading is 1.125 inches, 28.50mm, or 1.124 inches on the micrometer.
5 minutes of sanding with sandpaper held around pipe and rotated by hand. This was 120 grit garnet paper.
1.125" and 28.47mm and 1.123"
Sanding completed. At this point the BBB logo is completely gone. I cheated and used an electric drill to spin the pipe but still used only hand pressure and hand sandpaper. Once the 120 grit had shredded, I used some 320 grit and some wet&dry 600 grit wit CRC556 as a lubricant.
1.122" and 28.46mm and 1.122"
Final polished strip. Note the almost-mirror part, and the slight pitting above/below that where I didn't bother cleaning it up fully.
1.119" and 28.43 and I didn't get a good photo of the micrometer sorry.
Polishing was done with a small 3" fabric wheel and some car buffing cut-and-polish compound because that's what I had to hand. Not ideal.
Polishing a small piece of an anodised aluminium seatpost reduced the diameter by 0.07mm which is just less than the thickness of a softdrink can's wall, or 0.02%
That doesn't seem like a lot, but in terms of a seatpost, that's the difference between slipping and not slipping.
If your seatpost clamp has more flex then it may be able to take-up the difference.
However, simply touching the polished area had a significantly different feeling.
- Polished part was smoother because more of the surface is at the same level and is in contact with the hand.
- Curiously, the polished part "felt heavier" for unknown reasons.
- Holding the post in one fist like a hammer, the anodised part felt grippier because of the texture, and the polished part required a tighter grasp.
- The anodised part felt warmer than the polished part even though an IR thermometer showed them at identical temperatures. .
- It took 30 minutes to do this small band. Probably faster with better tools, but its going to total hours of time.