You'll probably want to take the rudder apart and examine the structure within, since there is a good chance it needs to be re-built. The rust stains are likely the rudder stock and webbing rusting...and it can lead to failure if you don't fix it properly.
Glassing over it isn't going to fix it. I'd point out that there is some sort of leak letting water into the rudder... and rudder webbing and stock has likely suffered from crevice corrosion.