Can I polish one section of a ceramic-coated car without recoating the whole car?

Yes. If only one localized area needs correction, such as a scratch or an etched bird-dropping mark, that area can be polished and recoated without stripping the entire vehicle. Polish slightly beyond the scratch or mark, remove all polishing oils with a panel wipe, then apply coating to the corrected area and blend it a little into the surrounding coated paint. The repaired section should bond to clean paint rather than to residue from the old coating or the polish.