Good Morning and welcome to DC.
The pan that has spots that will rust just needs to be "re-seasoned" The process is simple. Remove any rust. Wash it. Dry it. put a light coat of Crisco on it. Turn it upside down and "bake' in your oven at 350* for a couple of hours. Turn off the oven and let it cool. You may need to do this twice. Then use it for frying etc.(bacon, chicken, fish etc. not for brazing. The other one may just need a good washing with very light scrubbing and put into use. In one sense the "build up" is the seasoning. In another this "build-up" may need removing. Hard to say without seeing the pan. Can you post a picture of it?
There is only one Quality worse than Hardness of Heart, and that is Softness of Head.
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