Matt---about the best (cheap) way to do this, is use a coarse grit disc sander and take the paint completely off the area which you want to bond to. Cut a peice of mat (not woven cloth) slightly larger than the patch you have made----(about 1/2" larger all around), mix a small batch of resin and hardner, and use an old paint brush to cover the metal area with resin, lay the cloth onto the resin and "push" it into place with the paintbrush, then "paint" the inside of your patch with a bit more resin, then clamp the patch into place and leave it overnight.----Have all your clamps, etcetera ready to put in place before you mix the resin---you won't really have a whole lot of time to fumble around trying to get the clamps adjusted and placed, before the resin starts to "kick".----Your patch should extend about 2" beyond the rotted area onto good metal. Fair it in with body filler, and it will last 2 years to forever, depending on how much the tin-Gods like you.