Frustrated with inability to do body work!

Oh one more question, Keith noticed that my sanding pads had a lot of build up of the glue for the paper.

I have always left the paper on the block until its time to replace and i guess I shouldnt do it that way? Sometimes it might be weeks in between using them. I tried using thinner to remove this once Keith pointed it out, but it doesnt want to come of easily. Any suggestions and does this happen to anyone else? Is the cure to remove it each time your done for the day?
Thanks Rod