Bird muck etching

If the etching is not too far into the paint then wetsanding and/or machine polishing are in order.

Etching is often very deep, and thus impossible to fully correct. In order to attempt correction you need to polish the area using a polishing compound like Menzerna Intensive Polish, either by hand using a German Pad, or by machine using a polishing pad. Whilst polishing usually improves the look of such defects, it doesn't always fully correct them (you can try wetsanding, but you need a paint thickness gauge in order to do this safely). Sadly, etching is usually deeper than the thickness of paint that can be safely removed by polishing. Another option is to try and hide it to some extent by using a glaze such as Menzerna Finishing Touch Glaze; again, the results won't be perfect, but the fillers can help to hide the mottled effect a bit.