How can you replace a rotted kitchen sink cabinet shelf?

This is not an easy task, but I have been there and done that. First, make absolutely sure you have taken care of all sources of water entering the cabinet area. Check your disposer, water lines, cutoff valves, sink drain, drain pipes, dishwasher pipes, etc. I find that a brown paper grocery bag under each possible leak can help pinpoint additional drips that I otherwise might not have found.

Next, determine the extent of the leak. Depending on how far and how deep the damaged wood is, you may need a router, Dremel Rotary tool, and hammer and chisel. Begin removing the damaged wood. Protect yourself from splinters as sometimes these grotty areas can harbor really nasty bacteria. Don't be afraid to remove a little too much wood. Also remove any rotted, nasty wallboard.

Next step is to dry the area out. I find that a 100 watt lamp in a reflector base really dries things out in about 24 hours. You might want to put a fan there, too. I always saturate the area with Lysol spray to keep the odors down. After the area dries, you will likely see a little more wood to scrap or grind away.

Now, the filling step. There are several different materials that can help. One is Bondo repair putty. This is a two-part material that you mix and quickly press into any gaps or divits. Another filler is epoxy, which is also two-part stuff but typically is more expensive than Bondo. In one problem area, I also had to custom cut a sheet of Plexiglas after filling the voids. The glass made a nice surface for the cabinet bottom while it was nearly impervious to moisture.

Finally, I paint all of the interior areas that may get wet again with a top-quality oil-base primer.