In-line Rotary Discs Choke

I would like to ask how to improve the leakage rate from Class III to Class IV for a MC-10/20 Type MOV Choke.
We already made the surface flatnesss to two light band.



Not really familiar with the model, size and pressure class you are talking about, but I would recommend to consult with the manufacturer. Otherwise it would be trial and error attempt with likelihood that you might alter its integrity.

Reason as follow, in simplified manner:
Tightness (better leakage rate) for all sealing components e.g. gasket vs groove, disc vs seat are depending on three main factors: seating stress, elasticity modulus and geometry (for example: if one applies force towards let say O-ring/round edge, is it going to be effectively sealed against let say chamfered chamber? Or is it better with wedge shape). Hope you get the idea for Plug versus seat.
I assume that this valve is actuated, and you already perform some method of lapping? Or in other words the angle of contact between Plug and seat is not OEM standard any longer.
Main mechanical constraint: stem/spindle strength, and brittleness of Plug/seat’s hard facing (Tungsten carbide/stellite/some sort)
Actuated: stem thrust is already pre-calculated by manufacturer considering seating stress and constraints above.

Therefore, by making it more flat, valve requires more thrust to generate higher seating stress in order to achieve current leakage rate (Class III). And even bigger force is required to achieve better leakage rate. Sometimes the direct consequence is that the Hard facing will start to crack.
Changing flatness or geometry might also have impact on its Cv.

Mirror finishing is good target for refurbishment attempt, but not necessarily will make the leakage rate better.

Above is a snippet.

Danlap, You’ve make a simple reply into a teachable moment - thank you!