Here in rust land it's customary to hit the flanges at least with an angle grinder just to get a fresh steel surface. Ideally it would be leveled but that's somewhat taken up by the gasket. 
On good jobs, we use a file and get it neat by hand. Any gasket gets some paste, aluminum gutter sealant works, as does perms flex ultra copper. We've used both many times. 
Remember to tighten everything. Up only at the end in unison. 

-craig

On Jul 21, 2013, at 12:26 PM, Edward Maglott <emaglott3@GMAIL.COM> wrote:

> Ahh, that sounds like what I need.  I'll read the directions but what
> is the preferred method of application?  Goop it up, assemble and let
> harden?  Iron wire as in steel wool?
> 
> Thanks,
> Edward
> 
> At 10:54 AM 7/21/2013, David Beierl wrote:
>> At 10:20 AM 7/21/2013, Edward Maglott wrote:
>>> mentioned "muffler paste" and I was searching for that on the FLAPS
>>> websites for it when I came across that copper seal.  So you used
>> 
>> Exhaust cement, muffler cement -- comes in a tube, toothpaste
>> consistency.  Knead the tube before use as liquid separates
>> sometimes.  Silicate-based cement that bakes hard, very useful
>> stuff.  Victor is a good brand.
>> 
>> With that and some iron wire you can patch surprising gaps.
>> 
>> Yrs,
>> David