One of the biggest mistakes I see my customers make when they come into our automotive machine shop is there use of the rotary gasket removing disc. These are made buy several companies and they claim that they "only remove gasket material". Many well intentioned people use these discs on there die grinders to quickly remove gasket residue from cylinder heads and manifolds. Its quick, easy and brings the sealing surfaces down to bare clean metal, but it also leaves small pits and valleys in the sealing surfaces, especially around edges and corners. Customers have brought me there cylinder heads that have been cleaned with those "safe to use discs" for resurfacing and in some cases I have had to resurface up to .003" more than needed due to the aggressive nature of those discs. In response to some of the complaints of these discs, several years ago the manufacturers of these discs offered a new, less aggressive and safer version of the discs that were not much better than the old ones. Many automotive dealers and shops have created a zero tolerance policy on the use of these gasket discs buy there techs. The problem is even worse for engines that use MLS gaskets (multi layer steel) that require a very smooth surface finish to seal properly. The damage caused buy these discs is easily noticed when resurfacing the head. The first pass on the resurfacing machine will reveal the flat areas and will show all the valleys created buy those gasket discs. The best way to remove gasket material safely is to use a carbide scraper. They are very sharp, machined perfectly flat and safe on aluminum. They are available from most shop supply companies for about 30 bucks. They will remove the gasket residue without removing metal from the sealing surfaces. You will still see the gasket stain but thats all that it is remaining, just a stain left from the old gasket.  As an owner of an automotive machine shop my advise is do not use these discs and if you have bring the parts to a machine shop to have the heads resurfaced properly before installing them.