I’m pretty certain you have to coat the inside surface for the best reflectivity. Coating the outside will give you a “mirror” but since the light would have to travel in and out of the glass the efficiency would probably be worse than the bare stainless steel.
Amateur telescope makers have been making their own first-order surface mirrors for a long time (including grinding their own spherical and parabolic reflectors and lenses - by hand!) so it may be helpful to Google that… For electro-formed reflectors, they (the manufacturers I’ve seen) list aluminum as the most reflective but most delicate of their coatings, and rhodium as a good balance of reflectivity and durability. I have successfully polished an aluminum measuring cup/baking dish to a pretty good surface, but it requires removing the anodized layer and sanding progressively with grits from 150 to 3000 before doing any polishing.
If you look at projector bulbs the coating is in the inside. Unfortunately they use either a parabolic or ellipsoid shape.
Even with a coating, the surface must be polished, since any imperfection would be telegraphed onto the coated surface.
