How do you prevent a new cast iron master cylinder from rusting?

I have been thinking about this too. My approach is going to be:

1--clean the bare metal/prep
2--paint with rust-converter for first layer
3--paint with either primer or just paint depending on the rust converter.
4--Really feeling crazy? clear coat? Seems a bit over the top to me.

I haven't decided if I'll buy a cast iron looking paint or go with semi-gloss black. I have had good luck wth buying appliance epoxy paint in black that's a bit thicker and more resiliant than just engine paint.

Just my .00002 cents worth

That would be the plan for an already-rusted MC.
Step 2 needs some rust for the converter to work.

Would epoxy paint be best for for a brand-new MC?

Don't forget, brake fluid is an excellent paint remover. Any coating you use on the MC will have to be resistant to brake fluid.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

Strip it of all working parts and send it out to be powder-coated.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

To prevent rust when I put a new (NAPA) MC on, I clean it well, used POR-15 metal prep, then POR-15 silver thinned with their special solvent, then Krylon primer, then Eastwood cast iron grey. Has looked great now for 8 years or so.