After every boil I wash my Ace boiler out with nothing but water and give the bottom and sides a good scrub with a stainless steel pan scourer. These things contain no grit or anything like that but really cut through crud. As you're just rubbing stainless on stainless they don't actually scratch the surface at all.
Actually I use on plastic FVs as well and they don't seem to even scratch the plastic either.