Pete, if you are still getting material buildup on a paper towel, it sounds like the pan is not clean down to bare metal. After you wash the oven cleaner off, are you giving it a good scrub in soapy water with some stainless steel wool? If it is still looking rusty after that, you will need to use some 50/50 mixture of vinegar and water to remove the rust. Then another soapy scrub before a final dry, coat with Crisco and an hour @500F in the oven.
Have you read through the sticky thread about cleaning at the top of this Cleaning and Restoration Board?