How to fix a blowing exhaust?

Where one of my end cans joins onto the Y pipe, I've got an imperfect seal and it makes the zorst pop like a bitch on the over run and yes the clamp is done up tight. The other day I took the can off applied a liberal dose of Holt's exhaust paste and reassembled. Result - Soon as I fired her up the gasses blew the paste straight out of the gap and I'm, back to square one. So how do I fix this? Is exhaust putty different from paste? Would that not blow out before curing? Should I be trying something else?
The gap is small maybe 1mm. Suggestions please.

take it to a welder and have it done properly

Loctite do a high temp copper based silicone, I used it on my 525 Akra pipes that wouldnt seal properly and it does work well!!!
Ive got my baffles stuck in with it at the minute to stop the annoying rattle, been in for 6 months no problem
http://www.bigjimny.com/store/index.php ... p_131.html

Yeah, I've been sucking air into my exhaust so I put some fire putty on the end caps but I wish I'd used a silicone based variant instead, I'm not sure it made much difference, I don't think it held when running.

When Superdans header came undone on a road ride the day before cadwell last year he lost the gasket too and so with no chance of getting one from a dealer we took a trip to halfords.

He got a silicone based blue stuff from there and much to our surprise it held up with no problems all day even while taking a track day beating.

It may of been this one http://www.halfords.com/webapp/wcs/stor ... yId_165691

Wasn't recommended for head gaskets but we were willing to try anything at that point for a temporary fix.