I agree with Frenchtown, don't use fix a flat, too messy, but check rim for nicks, etc, and seal the rim , bead to bead, spray paint might work, I have used this trick to seal alum wheels that were leaking thru the casting.
Also check to see if any flashing is on the tire, if so trim it. Sometimes I have to trim flashing to get the bead to fully bead up on wire spoke wheels.
I assume you are finding the leak by dipping the wheel/tire in water.
Jon