Removing gasket from AL heads

I use a putty knife with a real sharp edge to it to get the majority of gasket off. Then try to carefully work the rest off without gouging the metal. If the remnants are really glued onto the aluminum, take it to your local machine shop who has a head resurfacer and have them hit it lightly. Not enough to remove any significant amount of aluminum. Just enough to get it clean. Using a sanding block with some sand paper will work too if you're careful, but, you need to do it evenly so everything stays flat. It's easier to have the head resurfacer do it. That way you know the head is not warped.