String has a little localized rot, what to do? Video inside.
 

http://www.youtube.com/watch?v=3A7U6...ature=youtu.be

I don't know why I sound like 100 years old in the video but Here is the story.

1981 formula 302 LS Carpet wall at passengers feet drips water on front bulk which is not fiberglassed on top. This may or may not be an issue as it can dry out. The real problem is the fuel tank has a 2x4" board on either side that is not glassed. It rotted on the port side, starboard is perfect. Most of it is removed in the video. This board had contact with the actual string in two ways.
1. It was nailed into the glassed in stringer the rot went into the nail holes and a very small amount of rot occured on the string around the nail the wood is hard there if you push with a screwdriver.
2.The front of the unlglassed board touch the stringer near the bulk head. The wood is still hard but a little discolored. I will remove more foam to see it the rest of the bulk head is fine. I have not interest in tearing the boar apart to be PERFECT if I can repair the rot in a good manner.

Dig it out and foam it with some 16lb 2 part.
http://www.uscomposites.com/

I am pretty new here, so are you saying cut out any rot (holesaw etc) then use this foam? http://www.uscomposites.com/foam.html to fill it. Then fiberglass it?

Thanks for input.

That's what I did. Do as little damage to the fiberglass around the wood as you can, pour in the 2 part and re-glass. The foam will be stronger then the wood around it and will fill every void no matter how small.

Also...find out where the water is coming in and stop it.

Rail fittings, cleats and windshield are good candidates for re-caulk.