For the epoxy solution, get a product such as System Three’s EndRot kit ($42.34 at Home Depot). This kit contains a few ounces of the materials you need to consolidate spongy wood and prevent future rot, plus two kinds of SculpWood filler: four ounces of a paste wood filler and 16 ounces of a thicker, putty-like substance. If you need larger amounts, buy the ingredients separately.
To repair the bottom of a door, you’d need to be able to work on the bottom edge, which involves removing the door and setting it on sawhorses or a worktable. Because the door is on the fourth floor, doing the work on the deck would save effort. Tackle the job when no rain is expected for several days, because the wood must be dry before you start. Check it with a moisture meter, which should show 20 percent or less.
Remove any rotten wood that crumbles from hand pressure alone. Then put on gloves, so you don’t get the uncured epoxy on your skin. (With repeated contact, you could become sensitized to it and develop a rash or blisters.) Mix the consolidator, which comes as two liquids (resin and hardener) in the proportion recommended on the label. Stir well, then brush the material into the wood. Use a liberal amount. If the rot extends well into the wood, drill holes and pour in the consolidator to ensure it reaches all the soft fibers. But plan ahead, because the epoxy stays usable for only about 30 minutes. The consolidator is thin enough to sink into the wood and harden it, creating a solid base for filling in the missing wood.
With the EndRot kit, it’s not necessary to let the consolidator cure before applying the SculpWood putty. The paste filler works for surface divots, while the putty is thicker and is suitable for replacing bigger sections of missing wood. You can shape it with a putty knife, and when it dries, you can sand it to re-create the original shape. The putty doesn’t shrink as it dries, so you don’t need to overfill to wind up with flat patches. A coat or two of fresh paint will make everything blend in.
The epoxy option works well when a section of a window frame or standard door is rotten. But a sliding door presents a much bigger challenge, because the bottom edge has rollers that fit over the track. Although it might be possible to drill into the cured putty to create recesses for the rollers, replacing the main section of rotten wood with a new piece that’s already cut for the recesses probably makes more sense. For this approach, you’d also need a string of dry days, and you’d also start by taking down the door, so you could work on it while it’s horizontal. Unless you’re a skilled woodworker, you should hire someone who is. That person could decide whether to take the door to a shop for the repair or do it on your deck.
Note how the door is built: The vertical frame pieces run from the top to the bottom, while the horizontal sections fit between those verticals. Cutting all the way across the bottom of the door and patching with a single piece would look weird. Instead, you’d need to carefully cut out just the rotten part of the horizontal piece. (You could probably repair the bottom edge of the verticals with consolidator and the epoxy putty, because those edges are small.)
The best tool for cutting out the rotten strip of the horizontal piece is a plunge-cutting circular saw that runs on a track, which helps keep the blade perpendicular to the main surface of the door. Festool, a German tool company, popularized this system in the United States a few decades ago. Today, numerous companies offer track systems for other brands of circular saws. Kreg’s Accu-Cut circular saw track, for example, is $80.60 from Walmart. Start and end the cut where you won’t nick into the vertical frame pieces of the door, then use a sharp chisel or an oscillating multi-tool to tidy up the ends.
Before gluing on a patch piece of wood, cut the recesses for the rollers. A router with a template would work well for this. You might also want to use a biscuit joiner to cut mating slots in the door and the replacement piece, then slip in wafer-like “biscuits” to help align the parts while you glue them together.
Unless you plan to do the work yourself, get an estimate for what this kind of repair might cost, and compare that to the price of a new sliding door. You probably wouldn’t need to replace the whole door assembly, just the door, unless the door frame is rotted, too. If you run into challenges finding a skilled woodworker to patch the door, know that it’s probably easier to find someone to replace it. Home centers and window and door companies are good places to start; window-replacement companies offer different services. A company that specializes in installing sliding doors might even go to your home to measure and try to determine the manufacturer.
Whatever solution you choose, address the water issue that caused the door to rot. If you don’t change that, you’ll be faced with the same problem in a few years.
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