Say I have some 230V and 12V traces that should be double isolated. I look up the clearance and creepage, and get the proper isolation distance for pcb layout.
Now for the question, often I see this gap left without soldermask, what are the advantages/disadvantages?
I would think that the path would be less prone to creepage with normal soldermask?

 

The solder mask is typically avoided as it may have micro voids,etc.. that can trap contaminates/moisture,etc... that could cause arc over/reduction in the creepage/clearance distances..
Quite often people also use a physically routed (milled away) gap that is applied post solder mask to ensure consistent gap/help with preventing carbonization,etc... as well as allowing greater surface distance vs air distances

 

For separating such large voltage differences I have used separations of at least 1/4 inch (=7mm) or more if possible. It is safer to have more than the minimum allowable gap if you can.