as you saw I made a typo there and have corrected it now.
Stainless steel is much more expensive than mild steel, why would you buy stainless and then paint it?? There can also be a problem with crevice corrossion on stainless steal threads. If you use Stainless Steel exposed to Salt Water you have to use the correct grade, some grades of Stainless Steel (there are hundreds of different Alloys classed as Stainless Steel) many DO corrode in Sea Water.
If the paint coating breaks down there could be very intense local corossion.
There has been discussion here on Galvanic / Electrolytic corrossion before, basically if you want to avoid corosion you use materials that are close together in their Chemical reactivity.
There was an old English definition of an Engineer "Someone who can make something for 20 pence that any fool could make for a pound". Painting Stainless steel would be like Silver Plating solid Gold jewelery.
Same problem with Stainless bolts and cast iron. The Bolts are always cheaper than the fittings they are bolted to, so why would you use Stainless Steel bolts that will not corrode themselves but will destroy the expensive fittings they are used on??
Galvanised or BZP (Bright Zinc Plated) Bolts are good, the zinc will corrode before the mild steel of the bolt or the fitting, and they are far cheaper than stainless.
It is always better to ask a stupid question than to do a stupid thing.