Repair Rotted Wood
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Wood, especially exterior wood trim molding, can become saturated with water and rot in isolated spots. To save the cost and work of replacement try to repair this damage with a wood filler like Bondo®. This is a two–part putty which when mixed, will set to a rock–hard patch that can be sanded and painted just like real wood. It can also be drilled, cut and shaped with the same tools used to work wood.
Remove the Rotted Wood
Before you start make sure the wood is completely dry. Dig all the soft, loose and rotted wood from the affected area. Coat the sound wood that remains with a liquid hardener. Bondo also makes a product —Rotted Wood Stablilizer— for this purpose.
Reinforce Deep Repairs
If the repair is deep, drive nails every few inches to reinforce the patch. Use nails with a broad head and drive them to just below the finish surface of the wood.
Build a Form
For deep repairs build a form around the damaged area using small pieces of thin plywood or similar material.
Mix the Filler
Follow the directions on the container to mix enough Bondo to fill the patch area. If the patch is very deep it may take two or more applications to fill and smooth the surface. Let the putty harden between applications.
Fill the Wood
Once the putty is mixed, work quickly to fill the patch before it can set, usually 5 to 10 minutes. Squeeze filler into small crevices and cracks using a plastic bag with a small hole cut in one corner.
For deep repairs pour the filler while using a putty knife to push it into all sides. On vertical surfaces the filler will run off and must be held in place with a piece of plywood or other rigid, flat material.
Overfill the Form
Fill the form to overflowing with putty until it is bulging at the top, but not falling over the edges of the wood.
Shave the Filler Surface
As soon as the filler has set, remove any form and use a paint scraper to shave off the excess and level the surface. Work before the putty can harden (20 to 30 minutes).
Apply a Finish Coat
When the missing wood is filled out, mix some filler and apply a thin coat to cover any pits or voids in the surface.
Sand to Finish
Let the putty just begin to set and then use the scraper to remove the excess. Immediately sand with 80 and then 120 grit sandpaper to smooth the surface for painting. Use a sanding block to keep the surface of large patches flat and square.
When the patch has set completely and is no longer warm, prime and paint with the rest of the wood.
