The JB Weld is a nightmare on 4 stroke mufflers. The Slimline is fabricated from thin wall tube that is joined with an aluminum brazing alloy. In orde for successful repair by rebrazing the surface must be very clean. The JB when heated gets right into the aluminum and must be completely removed . There are very few adhesives that will give a suitable bond in this application.

You might consider contacting Slimline and explaining the situation. It can't hurt.
If they can't help send me a PM and I'll see what I can do to help.