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How To Fill Exterior Rotted Wood

Wood is a popular material for building homes because it is strong, renewable, and sustainable. But no matter how strong it may be, exposing it to external elements leads to damage. If you have exterior rotted wood and you're wondering how to fill it, you're in the right place! We have researched wood decay and put up tips to address your filling concern.

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To fill exterior rotted wood, follow these simple steps:

  1. Cut out the rot.
  2. Scrape off old paint.
  3. Apply wood restorer.
  4. Mix the wood filler with hardener.
  5. Apply the filler to the wood.
  6. Let it dry.
  7. Sand the wood and paint.

In this post, we will discuss these steps in depth. There are also many things to learn more about proper exterior wood care, so keep reading.

Window frame in need of replacement, with rotting wood and peeling paint - How To Fill Exterior Rotted Wood

Filling Exterior Rotted Wood

Termite damage and wood rot showing beneath siding

Exterior wood is susceptible to decay. As long as you nip it in the bud, salvaging wood is easy and budget-friendly. You can use a wood filler for exterior rotted woods by following the instructions we have provided here:

Materials/Tools You'll Need

  • Claw hammer
  • Router with V-shape grinding bit
  • Utility knife
  • Wood filler
  • Spatula
  • Sandpaper
  • Paint
  • Wood primer 

1. Cut Out The Rot

The rot will continue if you don't remove the decayed part. Use the claw hammer or utility knife to cut it out. Use the router with a V-shape grinding bit to grind down to solid wood to ensure that the wood filler will bond properly.

2. Scrape Off Old Paint

This step is essential as many fillers or patching solutions won’t stick if there is existing paint. Use a sandpaper or paint scraper to remove paint residue on the wood’s surface. Dust off the surface before moving on.

3. Apply Wood Restorer

Wood restorers solidify spongy woods. Apply the wood restorer and wait for it to dry and stabilize.  Take note that some wood restorers only need a couple of hours of curing, some may need to rest overnight. So, be sure to read the directions for the application.

Click here to see this wood restorer on Amazon.

4. Mix The Wood Filler With Hardener

This is done when you are using a 2-part filler.

Prepare a non-porous mixing surface. Place the filler on the mixing surface. While the hardener cap is still on, knead the tube well to blend its content. Put about 3 inches of hardener across the filler that is already on the mixing surface.

Using a spatula or spreader, mix these two components for about 2 minutes or until you see the two colors blend into one.

Be conscious of time when dealing with mixed fillers. There’s only a short span of time before the mixture cures so only mix small portions at a time.

Click here to see this wood filler on Amazon.

5. Apply The Filler To The Wood

Using the spatula, spread a thin layer of the wood filler onto the wood. Keep in mind that you’ll need to apply strong pressure for maximum adhesion. Additional layers are also required for the desired thickness.

6. Let It Dry

Before moving on to the next step, allow 30 minutes to 2 hours of drying time. For deep repairs, drying time may take longer.

7. Sand The Wood and Paint

Smooth the area with sandpaper and then feather the surface to remove dirt. Apply primer and allow it to dry. Once dry, you may now paint the wood to your desired color or match it with the rest of the area to conceal the repair.

Other Ways To Repair Wood

Apart from filling rotten wood, there are other ways of repairing decaying wood as not all damages are the same. 

Using Epoxy

You can also patch the rotted wood with epoxy. In doing so, you must follow the first two steps that we discussed earlier.

After removing the rotted part of the wood and scraping off the paint, you have to apply a bonding agent to the wood so the epoxy sticks better.  

You will then have to mix a 2-part epoxy with your spatula on a non-porous surface. Mix the two until you see a uniform color. You have about 30 minutes to work with epoxy before it dries out.

Apply a generous amount of epoxy and push it to the area of the wood you are repairing. Wood expands and contracts and epoxy flexes with it so they won’t separate.

Scrape off excess epoxy with your spatula. When the surface is even out, you may leave the epoxy overnight to dry before sanding it and putting a primer and paint.

House renovation. A painter removing old paint with a scraper.

Using Wood Patch

If the rotten part is too soft to be filled with epoxy or wood filler, you may remove it and patch it with new wood.  

A set of old, cracked, peeling exterior deck board planks on a home with one that has warped alot and is curling up from exposure to the elements outside.

  • Cut the rot with a saw. 
  • Measure that cut to create a replacement piece. Make sure that the new wood fits perfectly into the rotted-out area. 
  • Apply wood glue and place the patch. Let the glue cure.
  • Drill crews to the patch to hold it in place. The crew should be long enough to reach the base.
  • Sand excess glue off the patch before painting it.

What Is Wood Rot?

Moisture is the number one enemy of wood. When wood is exposed to water, fungi start to grow. Wood-rotting fungus digests moist wood, causing it to lose its strength.  

Wood rot can come in the form of brown or white rot. Brown rot is characterized by shrinking, cracking, and showing a brown discoloration of the wood.  This rot is dry and crumbly. White rot attacks living and dead trees. You'll know that it is white rot when wood becomes spongy and turns white or yellow.

Is It Wood Rot Or Termite Damage?

Sometimes, it is hard to tell if it is brown rot or termite damage because they almost look the same.

Decaying wood is spongy and breaks off in cube-like patterns. It may crumble when touched.

Termite damage may appear normal on the outside but has tunnels inside.  There are small holes that appear in the wood. When tapped, the wood sounds hollow.

Window frame in need of replacement, with rotting wood and peeling paint

Termite damage may be trickier to repair. Unlike wood rot, where a part of the rotted area can be repaired, termite infestation requires the removal of the colonies before removing or replacing the compromised wood. 

What Happens to Untreated Rot?

If you don't stop the water source from wetting wood outside your home, the latter will deteriorate and will result in permanent structural damage. If you spot a small area of rotten wood on your porch, doorway, window sill, or anywhere outside (or inside) your house, immediately treat it.  

Rotted Wood Prevention

Aside from stopping the water source, it is also imperative to protect the wood. Water sealing exterior wood is always a good idea. Don’t allow rainwater or snow to accumulate or sit around the house exterior.

Boric acid, when combined with insecticides, may be used during construction to prevent future rot. It protects wood from fungal and insect attacks.  Borate-impregnated rods may be inserted into the wood to protect it from moisture.

When Should You Replace Rotten Wood?

We have talked about repairing wood rot. Yes, we can repair wood, but only up to a certain extent.  Sometimes, fillers just won't do. Ask yourself these questions before replacing wood:

  • What is its structural significance? 
    • If the rot is on areas that will compromise the strength of the foundation, it will be better to replace it with a new one.  
  • How big is the damage?
    • If the wood has cracks, discoloration, and has become soft, means it has significant deterioration. Consider 15 to 20 percent of damage as a sign that the wood needs replacement, instead of repair. 
  • How much will this cost?
    • In connection to the second bullet, small damage cost less to repair than replace.

Are Wood Putty and Wood Filling The Same?

Preparation woodwork. Close-up details Putty knife in man's hand. DIY worker applying filler to the wood. Removing holes from a wood surface. Application of putty.

Wood putty and a wood filler are both used for repair, however, there are scenarios when one is more applicable to use than the other.

To smoothen small surfaces caused by screw or nail holes, scratches, and the like, use wood putty. On the other hand, if there are gaps and bigger holes to fill on walls and on decayed wood, it is better to use fillers. 

These two also differ in the timing of application. Putty is used after staining or varnishing a wood piece. Wood filler is applied before your finish or stain.

Although these two are similar, there is not one that is better than the other. Both are useful in different situations. 

Final Thoughts

Repairing rotted exterior wood by using fillers is an easy task if you have detected the issue in its early stages. The success of your project also lies in the use of proper tools and following the steps properly and chronologically.  

Before you go, check out these articles related to our topic:

How To Repair And Smooth A Splintered Wood Deck

How To Fix Holes In Hardwood Floor