Sharpening the blades on most plastic props will not help you "slice" through weeds, and will only weaken the edge of the blades, allowing them to become damaged easier the first time you hit a stick or something in the water. That's just the nature of how the prop is designed and built. More than likely, you're trying to sharpen the blades on a Motorguide prop, aren't ya? Motorguide fans - please don't take any offense at all. I'm not badmouthing MG - they make a fine motor - I have just found that MG props don't shed and cut through weeds anywhere close to as well as the MK Weedless Wedge props. If you have a Minn Kota with the Weedless Wedge prop, then I would be supprised that it is not doing the job, and would suggest raising the motor a bit to get through most of the really bad stuff. If it is a MG prop, you may want to consider going to the metal Machette prop, or trying one of the aftermarket props available on the market - maybe a Kipawa prop.
All the best,
Glenn