Our exterior door leading to the patio has developed rot at the base of the main support. While talking to a contractor friend, I found that just simply replacing this with a new piece of wood is nearly impossible since it has screws that come up through the metal threshold and it needs those for the proper support. The hinges on the door are attached to this piece so the door would sag if this is done improperly.
I had the idea of designing a 3D model of the wood and the threshold base to fill with waterproof epoxy. The then epoxy will harden onto the screws coming up and also I can add screws up into the solid part of the wood after cutting away the rot.
Step 1: Prepare the 3D Model
I used a caliper to get exact dimensions of the wood and I did my best to line up the bottom of it with the threshold. It ended up being a little overkill with the small bumps but the large rail was useful to lock it in. I use tinkercad to design the model and printed it on a 3D printer a group of us own. You're welcome to use my design or look at it for inspiration.
Step 2: Get Supplies / Tools
I purchased waterproof epoxy from amazon called Amber T88 (System Three 1100K16 Amber T-88 Kit, 1 quart Bottle):
I also purchased grey model clay from Michaels to use at the base. It is waterproof and will not let the epoxy leak out the bottom.
Disposable cups to mix the epoxy
I use a Ridgid Multi-Tool but you could use anything to cut away the wood rot
Step 3: Clean Up the Rot and Position the Model
After cutting away the rotten wood at a point where it is solid and dry (I didn't cut it cleanly across for better grip of the epoxy), add 2 drywall screws up into the wood. Then I applied the grey model clay at the base. I found that I couldn't quite move the model into place with the screws coming up through the threshold so I had to cut them down a little and bend them. There was still plenty to work with for the epoxy to grab onto.
Step 4: Mix the Epoxy and Pour It In
I drew a line across the same level of 2 plastic cups so I could mix the same amount of the epoxy and hardener. I used some plastic sticks to mix them in the bottom of a milk carton. It says that you have 45 minutes to work with the epoxy after mixing it. I found this to be very true with a small sample. It was basically liquid and then went right to solid almost instantly at about 45 minutes.
I poured it in until it reached the top and then snapped my slider door into 2 pieces and put the lower piece in so I could fill it higher. I continued to push it in until it was coming out and slid the last piece into place. Then I used some of the clay to wrap around all of the edges to keep it in. I struggled with this a little and ended up using some duct tape with a clamp to get it to hold on tight.
I let it harden for 2 days (1 would do).
Step 5: Clean Up the Plastic and Epoxy
I used the Ridgid multi-tool to cut away the models plastic and then sanded it smooth. I used spackling to clean up any rough spots and then sanded that smooth.
Step 6: Finish With Painting
After painting it, we're all done. It is a solid, strong supported door now without any rotten wood. As long as I keep up with paint then I shouldn't have to worry about any rot on the rest of the trim. But the base of this support piece in the middle will never be rotten.
I have another door with a lot worse rot in the same situation and it goes up a lot higher. I won't be able to make a 3D model so I am considering something with cutting a model with acrylic and my new laser cutter. I try and post updates and pictures if I follow through with it.