1. Are you measuring with a profilometer? If so you are measuring wave and lay, not microscopic surface texture. Anything below about 30microinch is in voodoo land.
2. The EP may have removed a distorted layer of material from a poor machining job and revealed the underlying texture.

Look at this under magnification (at least 30x) and see if it looks free from details. It may be lumpy or wavey but should not be covered with small marks. If this is the case then they did there job perfectly/

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P.E. Metallurgy, consulting work welcomed

Does it need a 16 micro. Suggest centerless grind. Or does need to look pretty.

Hi, it needs a 4 - 8 uIN finish to reciprocate in an energized lip seal (Variseal, etc)

electro polish is mainly used for stainless steel parts. I suggest to grind and polish, talk to a grind shop

Thanks guys,

Ed, here are the pics under 30X....

https://res.cloudinary.com/engineering-com/raw/upload/v1640121607/tips/piston_surface_finish_images_rhfbkt.docx

Thoughts?

Micromesh comes to mind. The make up to 12000 grit which they say is material removal rate, but produces a far finer finish. I've used it for polishing steel to a mirror finish for a long time.

Thanks mfgen and 3D...

would hard coat anodizing improve the finish at all?

no and it weakens the part, I forget the specifics, it's used as a protective coating and to make them look pretty

What concerns me is that the marks look like the run lengthwise.
If they were machining artifacts they should be circumferential.
So are they related to the microstructure of the material?

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P.E. Metallurgy, consulting work welcomed

Mfg - thanks
Ed - they were turned in a lathe prior to EP. So you’re saying it could be the material?