I've been creating hand-drawn maps for a while now, and I thought I'd gotten the hang of it, but a new problem has reared it's head. I stretch the 300 gsm paper before I age it, but several times in the past two months by the end of this the paper starts pulling itself apart as it dries. It doesn't happen every time, but it's disconcerting to say the least - any ideas on how I might avoid this?