The coating is a a chemical bath as part of the stainless steel processing. Its what makes the metal scratch resistant. I've never heard any rabanim say it loses its kashering status from this and I've discussed it with prominent poskim. Some newer types have an additional poly based coating. Presumably that would be the same as any sealed stone countertop.
Regarding irui kashering from a kli rishon while it's still al ha'aish, it's brought down from R Shlomo Zalman in halichos Shlomo that it's as good as even meluban.
I used a steamer that sprays boiling water as well to to kasher at thege homes of several prominent rabanim they praised it highly since it has the ability to hit every spot so easily including the under side of the sink rim and the underside of the faucet. They all said that it was kashered lechatchila and no need to cover (though I know one covers anyway ) they even said it would be better than kashering with a torch (that's how I had previously kashered by them) since most are just torching to yad soledes which isn't as pashut (though in my experience at yad soledes the sink insulation starts smoking which seems to indicate that yad soledes meiver hasheini and Kash nisraf are one and the same like the mordechai writes)
Last thing, all the morei horo'ah I consulted with thisnyear were very insistent that WE ARE ADVISING people they can kasher plastic and synthetic materials this year.
Maybe
@aygart can chime in on that last piece...