The white layer seems to be dependant upon pulse energy, reduce the pulse energy and the layer should reduce, the idea of roughing out first is probably a good solution with the with the white layer reduced in the light finishing pass, use of kerosene instead of de-ionised water may also help.

Thank you Sir.

Dear Daniel,

The thermal energy discharged in machining zone mainly governs the white layer thickness. Controlling machining parameters like discharge duration, voltage, pulse current, the white layer thickness can be minimized or controlled. The tool material will have less influence on white layer thickness.

You can read more information in following papers.s Article Experimental research on EDM of AISI O1 tool steel and study...

Article Modeling of White Layer Formation in Electric Discharge Mach...

Article Structure and Composition of the White Layer in the Wire-EDM Process

Article White layer thickness prediction in Wire-EDM using CuZn coat...

Conference Paper EDM performance is affected by the white layer

https://www.sciencedirect.com/science/article/pii/S2452321618300817

Article Prediction of white layer thickness and material removal rat...

I hope that these articles gives you better insights.

Best regards.

Deepak

Many, many thanks Mr Deepak Rajendra Unune

Most welcome Daniel.

The white layer is actually a double casting layer in which the material is first melted and partially removed from the spark by dielectric and part of the melt is solidified again and placed in the same place as before. Therefore, heat energy and spark energy affect the amount of this layer and also the process of washing with dielectric can affect the thickness of this layer.