adjacent poly's overlap

[Brent Pedersen]

hi, i'm trying to display the outline's of a polygon layer where each  
polygon is color code by a CLASS type.
the problem is that most of these polygons share a border.  the  
problem is visible here:

>> http://giifweb.cnr.berkeley.edu/fire/geoFRAP/060_247802000000.png   
>> (ignore the circle)


i tried (after some postgis help) to use buffer, but that is slow and  
not exactly what i want. is there a way to get the inner boundary of  
a polygon to display in mapserver to avoid this overlap?

thanks,
-brent