Basically, there's not much to talk about if we create this thread.

For do-it-yourselfer's, we use Flitz or Mother's Mag Polish on factory stainless finishes to get them to a high polish, almost "bumper chrome" look.
Then for those who want to reverse the high polish back to factory, we use 3M scotch-brite pads.

If one wants to go matte or glass bead stainless then that should best left to a professional shop.

For blued guns, there is not a whole lot for the DIY to do. Because odds are the finish will be ruined. For high polished blue, that all begins at the factory. If the DIY wants to duplicate this, good luck because most likely you'll ruin the finish. (A DIY high polish blue that rivals the factory finish is like the Holy Grail.) The only thing a DIY can do is to polish the existing blued finish with Renaissance Wax or a Carnauba car wax.

If you want to matte the blued finish, this is also best left to a professional shop.
Why? Because of the expense and trouble one goes to obtain a blasting booth, the media needed, the stripping of the frame, the masking of certain areas, the extensive cleaning afterwards to remove every piece of grit. And then one has to have the apparatus and chemicals to HOT re-blue the gun.
Notice I said HOT, there is no magic COLD re-blue out there.
Cold blue formulas are for small touch-up jobs only.

I forgot for those that like the matte black finish there is always the DIY parkerizing kit or the spray on, bake in the oven finish. Knock yourself out! But I like to call this "Bubba" or garage gunsmithing.