Brass slivers are non-magnetic and really difficult for the hospital folks to get out of your eye. Remove the filing or grinding marks with successively finer grades of abrasive paper. Be sure to get all the marks out before going to the next finer grit. You need to seal the joint between the blade and guard so moisture cannot get in and rust your tang. Soldering is the usual method.
Silver solder is the strongest, but requires a high temperature.
If your joint is good and tight, you can probably use soft solder. Remember to use lead free solder if the knife may be used on food. Clean the parts of all grease and oil. You want to heat the joint without heating the blade to the point you ruin the temper. It helps to wrap the blade in a wet paper towel.