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Cedar Siding Knots have fallen, leaving large holes that need filling

bridon's picture

Fixing to start work on a house with vertical cedar siding.  Some knot holes (actually quiet a few) have popped out and squirrels have capitalized on the problem.  My problem is how to fill them.  Several searches say: Bondo will not expand and will eventually pop out and Minwax wood filler will do the same. There is a very expensive epoxy product out there too.  Does anyone have any tried and true solutions for this repair?  Also, can anyone lead me to a repair solution using a template and router to cut out a plug of bad area and reinsert a new plug, somewhat like the 'footballs' in plywood.

Certainly you could use the (post #187226, reply #1 of 9)

Certainly you could use the template and router idea -- it's something you can easily set up yourself without any special supplies or equipment.  Just make your own templates out of sheet plastic or Masonite.  You can buy a set of interchangeable collars (for doing inlays) for the router such that with the wide collar on you cut out the opening and with the narrow collar you cut the plug, using the same template.

For example:

Such kits may be a little lightweight for cutting a lot of plugs, though, so you can just make multiple templates -- one large for the plug, one small for the hole -- and use a heaver router bit.  WIth a little cleverness (and the right set of router bushings) you can cut the small template from the large one.

The other idea would be to use hole saws and plug cutters, but I don't know that they make plug cutters large enough for the typical knothole.  (Though on Googling I did find a 1.5" plug cutter.)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Cedar siding knots (post #187226, reply #2 of 9)

Thanks for the reply.  I suppose the router option will turn out to be the most professional looking repair, outside of replacing the entire board. 

I can't count how many (post #187226, reply #8 of 9)

cedar knots I have filled with bondo... and I haven't had any call backs to fix any that have fallen out.

Slide a piece of tarpaper behind the hole and overlapping the lower course. Bondo doesn't move much in relation to the siding (but then, neither do the knots, which is why they finally fall out).

We may drive screws partially into the sheathing or push in some expanded lath... anything to key the bondo.

Color the bondo with some of the house stain when you mix it. If the cedar isn't stained, put some in that matches the other knots, if there's any left<G>

Mix the bondo hot and in small batches, and mix till it's like bread dough. Don't smear it around.


Or you could make bondo footballs.

The Village Woodworks, Inc

Chapel Hill, NC


We'll have a kid Or maybe we'll rent one He's got to be straight We don't want a bent one He'll drink his baby brew From a big brass cup Someday he may be president If things loosen up

I'd patch them with thin (post #187226, reply #3 of 9)

I'd patch them with thin metal plates, caulked and tacked down. 

The router trick would work better though.  I'm assuming the cedar is 3/4" or more thick, but the patches would only need to he a half inch thick. Epoxy them in. 

Another rough idea would be to use a drill with a forstner or similar bit to drill round holes with flat bottoms partly through for round plugs.

Filling knot holes in cedar siding (post #187226, reply #4 of 9)

I have the same problem with knot holes in my cedar siding. One of the holes is approx 21/2" x 1/12" and the birds have pecked away the insulated board behind the cedar...there is now a hole that extends all the way to the plywood siding... approx. 41/2" deep. Is there another way of filling knot holes other than using a router with plugs or metal plates of some sort? I was hoping there was a type of wood filler on the market, or a combination of spray-in insulation from a can and wood filler that will do the trick.  The other holes I have appear to be only as deep as the cedar siding...the birds have not pecked through the insulated least not yet.


Rick M.


You certainly can use wood (post #187226, reply #5 of 9)

You certainly can use wood filler.  If there's no backing behind the hole to help mold the filler (and you can't easily access the back side to install a backer) there are tricks using cardboard and string/wood strips and wire, etc, similar to those used for drywall patching.

Basically you first clean out the hole well (a rotary rasp in a drill might be the trick here), install the backer (if not already present), and then pack in the filler.  You can use Bondo or one of the "Plastic Wood" type fillers, or even tree wound cement -- whatever "feels right" for the size and shape of the particular hole.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

You also could use paper and (post #187226, reply #7 of 9)

You also could use paper and trace the outline of the hole, then transfer it to a piece of wood. Saw out the patch and put it in the hole.  Use gel epoxy or even bondo to adhere it in place.

Just an FYI - If knots have (post #187226, reply #6 of 9)

Just an FYI - If knots have fallen out it means this particular branch was dead. When buying any lumber with knots all the black knots are from dead branches and will eventually fail. I put a generous dab of construction adhesive on the knots backside before installation and this hold the knot in place. 

Did you try "GoodStuff"??? (post #187226, reply #9 of 9)

This may sound goofy but it has worked for me.  I take a can of the expanding foam (branded GoodStuff) and I fill the holes.  A few hours later there is the overfill from the foam expanding, so I take a knife and cut it to match the flat or the edge I need.  Then I stain it and it looks pretty good from the street.

I'm a homeowner not a contractor so I don't really know if people would like the finish.  I guess you could caulk it before you stain it to make the surface smooth.