I just took my very seldom if never used bathroom sink drain apart because I had some water that backed up into it. When I took the U-joint drain off there was a white solid (almost salt looking) blockage at the bottom of the U... what is this and how does it form? After I removed that the water drained better, but it must have hit a blockage further down... now I'm stuck... should I try Drano?

Thanks

Hold off on the chemicals for now. Your back up is caused by a blocked drain line further down the system. This must be a main floor sink with other fixtures higher up, maybe a second floor. That U-joint is a P trap and it was filled with whatever was in the water that backup up from another fixture. Remove the P trap and rod straight into the pipe coming out of the wall. You will hity a 90 elbow a few inches in so keep twisting, I use a drum auger on an electric drill.