What brass to paint

Hi I am in a quandary of what brass parts to paint or should I polish them all.
Are there any parts compulsory to paint? Your comments please.

It's your engine so nothing is compulsory but I have noticed what in my opinion are over polished engines.

I would recommend painting anything that would have been cast iron or steal on fullsize. There will be pieces that have been made from brass/gunmetal on the model purely for ease of manafacture. A glaring example on the 4" is the blanking plate behind the blowdown valve to cover the oversize hole. No such thing exist on fullsize but I have seen engines with this gleaming. I think the pump is best painted but only after testing. The bottom line is that on most agriculturals there would not be much to polish beyond the fittings but as always there are exceptions.