If it is leaking that much I would recommend you just replace with a new all-metal replacement.
www.radiatorworld.com sells an all copper/brass replacement for $128 shipped. I went through them for mine and it went great, radiator arrived fast and fit great.
The leak fixer stuff can gum up your radiator, and even if it does fix the leak it will only be temporary. Just solve the real problem now to save trouble later.