There are various types of heating mantles, which require different cleaning procedures. Do you have a stand alone type (one with all the controls on board) is it a jacket type (just wraps the apperatus) common ball flask with glass cloth and plug for thermo controls. Is the shell S.S. or aluminum, Shiny or brushed. Are the stains on the cloth or the metal? For the brushed metal finish it is as easy as finding the closest match for scratch sand paper and have a go doing a stain removal sand, followed by a scratch match. The problems arise here when the stains turn out to be corosive, and you start out on a stain only to find the thin wall sheet aluminum pinholeing or crumbling. Thats addressed in metal shop. As far as the cloth covering the element, the stains are for sure going to be somewhat BAKED on due to use after staining. I've had better luck replacing the glass cloth. It is fairly straight forward, you use a high temp resistant adheasive to attach the new peice. Use a flask in the "cup" to prevent wrinkles, and it is VERY handy to use the flask to keep the cloth pressed then use a fine line marker to mark the excess to trim off. most of the adheasive is used around the top edge to keep the edge from fraying, and the fact that the element will burn the cloth AND the adhesive, not to mention the element itself is why you should have a transfer medium when powered up. S.S. rust is removed w naval jelly (med rust) dilute phosphoric acid (heavy rust) or CLR for light rust. S.S. or what some term as stainless steel is primarily used on clamp hardware that is permenently attached to the mantle body Be careful that the rust is all you wet with remover. Pitting on S.S. or aluminum is best veiwed as "stunning design characteristics" Otherwise fill with jb weld, sand when dry, and hope nobody notices. If they do, blame the least favorite entity that comes to mind and then cook, cook, cook!
floating on a gyroscope, thumbing our nose at the universe, what a ride!