I'll try and post some of the photos I took of my deck work. What you want to do is tap with a hammer find the soft spot. use a circular saw set to 1/2 inch or less. cut out a square. Chances are once it's open you will have to make the opening larger. Take a chisel and knock out the core. It's usually not hard to get out. After it's cleanout, mix up epoxy and micro spheres to a peanut butter consistency, spread it in the opening, wet 3/4 balsa with epoxy and place on top. Let it cure then fill in the gaps with more thickened epoxy. sand, sand then sand some more.
