All good suggestions. The only problem with starting out to do this is that as an initial experiment, you can't "guarantee" the results without the benefit of previous experience.
Varnishes, lacquers, clear coats in rattle-cans from the hardware store can be useful to "stop" the absorbency of a frame finished in gold leaf or dutch metal, to control the amount of "patina".
Oil paint is the perfect thing to tone a frame, that's what we're working with! Slow drying? Why? Add a siccative, and/or a rapid drying varnish like copal or damar.
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