With unknown previous welding, I would remove all of it and get down to the clean base metal. Use a carbide or HSS burr, not an abrasive disc, as they force contaminates into the aluminum. For really thick areas, preheating is nice, but you can also just let the TIG put in enough heat, it just will take a little extra time. If you get everything nice and cleaned, you can lower your AC bal which will give you a bit more heat, too. Start at around 30% and go down from there. Frequency will change how focused the arc is. Near the edges you might go up around 120Hz or higher for good control. To get more heat and a softer arc go lower. To emulate older welders run it at 60Hz. Be sure to establish a puddle before adding filler. Let the cleaning action work on the area and also bring the temp up, then bring in more current and start your puddle. You might want to do some practice on thick scrap material if all you have worked on is thin. Be sure to feather out the amps at the end of your weld and add a little more filler to avoid a crater crack. Let the wheel cool slowly afterwards, if possible throw a heat proof blanket over it to slow the cooling.