The J-B Weld comes in 2 tubes, white and black. It's mixed 50-50 and ends up an even medium gray. Takes 12-24 hours to cure to a very hard and adhesive patch.
The problem is either your stuff is "Kwik," or you didn't mix it correctly. I suspect the former. What you describe is typical of 5-minute epoxies--they never "dry" enough to be be sanded.