Breakdown on PCB versus air

Look at the following PCB material:

If breakdown is across the surface, then the material strength is irrelevant, and cleanliness/pollution/environment is critical. If you are providing 2.8mm as a standard layout guideline, this should meet most production requirements, providing that the boards meet the CTR of the safety documents, are cleaned and nobody leaves their fingerprints behind.

Check with your board vendor about the CTR for the process that you are buying - there is no guarantee that it meets the standards' assumptions, unless specifically stated by the vendor.

In dirty environments, slots will assist in avoiding accumulation of crud, but it's only a matter of time.

RL

usually brings about a permanent conduction channel within the bulk of the PCB. So you might have a permanent problem. If its a surface problem apart from cleaning well the surface try using a water displacement solution then add silicon greases to reduce the surface tracking . Breakdown voltages are considerably lower for dc then ac voltages. So if you can do anything about that try it out.

Cutting out a slot may bring in new problems like defects on the board which make it more vulnerable to breakdown coltages. The pointed areas from cutting also favor intense electrical fields. Thermal instability also contributes. Can you cool the board ?