I got myself last year an old beaten up cab, which was a Gravitar cab that has been converted a few times, and on which unfortunately the side art has been destroyed.
I noticed that one of the corners on a side was broken and chipped. Since the only product that was available at my local hardware store was plastic wood, I thought I could use this stuff to fill all the voids, scratches, holes and the broken piece of the corner. After letting the stuff harden for a few months (i didn't have time anyway to touch it before), I've tried to sand everything down but the results are not satisfying. The stuff on the filled scratches just went away, and what was used to joint and fill the broken corner just broke down, so I'm back to square one.





What should I do next ? Any suggestions would be appreciated, since for now I'm a bit discouraged at this outcome, and my woodworking skills are something near 0.
BTW, I'm in Canada.