How To Repair Rotted Wood

Updated Sep 3, 2024 | Posted Jul 30, 2024 | Product Advice | 1 comment

Sooner or later every decorator comes across the issue of wood which has (to a greater or lesser degree) rotted. So, I thought it was a good idea to put together a quick blog on how to repair rotten wood.

I’ve encountered it many times over the years – most frequently on old wooden sash windows and windowsills. These are increasingly being replaced with modern plastic alternatives, but there are still plenty around.

Another job I did recently was a big wooden playground renovation in a private garden, where I found rot in the top of some posts.

There will always be things made of wood, and decorators will always need to know how to repair rotted wood – how many wood-built “garden offices” have sprung up since 2020, and how long will it be before we’re all being called upon to repair parts of them once they’ve had a few years’ exposure to the elements?

Or maybe you’re tackling the maintenance to your own house as a DIY project, and you want to do a proper job. Carrying out the work correctly and with the correct products will ensure that your timber repairs will last.

 

Why the Correct Process is so Important When Repairing Wood

 

Before we get into the detail of how to go about repairing rotted wood, maybe we need to answer the obvious question: Why do we need to repair rotted wood?  All too often I see situations where other decorators/handymen/householders have simply spread some 2-pack filler over the area, sanded it smooth(ish!) and painted over it.

This does not solve the problem, it will return!  The wood will continue rotting underneath the filler, which will then come loose and fall away.  A proper repair therefore needs to be made to preserve the wood for the long term.  The ”old school” way to do this is to cut away a section of the wood and splice in a new piece of timber – this is clearly easier to do in some situations than others.

The new wood will of course last for years, but this type of repair can require carpentry skills that are beyond some of us, and there is always a risk of a small gap remaining between the old and new timber, where water could gain access and continue the rotting process.

These days, epoxy fillers create air-tight repairs which, when painted, are virtually invisible and certainly long lasting.

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How To Repair Rotted Wood

 

I’ll briefly list the tools and materials you need to repair rotted wood. I’ll then take you through the steps. I’ll end by explaining a little bit about the materials I use, why I use them, and where you can buy them.

 

Supplies

  • Zinsser Peel Stop
  • Wood hardener/epoxy primer
  • Two-part epoxy filler
  • Ordinary wood filler for the minor repairs
  • Your paint of choice (primer, undercoat and top coat)

 

Tools

  • Dust sheets if the area you are working in needs this protection
  • Scrapers, and sandpaper of various grades, or dustless sanding system
  • Scrapers, knives, chisels, possibly even a router for removing the rotted wood

 

Steps

 

Step 1 – Cover or move anything that needs protection

It’s much easier to take an extra five minutes up-front to cover or move anything that needs protection, than it is to try to remove a stray blob of epoxy filler which has cured on the rear wing of your client’s BMW (not speaking from experience, of course!).

 

Step 2 – Remove all Loose, Flaky Material

Use scrapers and coarse sandpaper to remove as much loose and flaky paint as possible.  There are two obvious reasons for doing this; ignoring it and simply painting over the problem will solve nothing, the old paint will continue peeling and you will have to re-do the job properly in the very near future.

But secondly, it gives you a much better view of what state the wood is in underneath.  Be thorough at this stage, and once all loose material has been removed, sand the edges of the remaining paint so that the areas where there is no paint, and the areas with old paint remaining, blend smoothly into each other without any “ridges” or “steps”.

Once you are confident that this stage has been completed thoroughly, it’s a good idea to give all the edges of the old paint a coat of Zinsser Peel Stop; this clear primer will help to “stick down” any paint edges to reduce the risk of the old paint flaking in future.

stabilising paint before repairing rotten wood on a windowsill

Step 3 – Remove All the Rotten Material

Any rotten wood is going to feel soft, possibly spongey or crumbly.  Use scrapers, knives, chisels, even a router if you have room – whatever is easiest to get rid of all the soft material until you are back to hard solid wood. Brush or vacuum out the holes to remove all remaining residue.

 

Step 4 – Treat the area with a Wood Hardener or Epoxy Primer

This step effectively reinforces any slightly rotten wood which still remains, creating a more solid substrate for subsequent filling to bond to.  You can use something like Ronseal Wet Rot Wood Hardener (surprisingly difficult to say after a couple of beers), or recently I’ve been using Oxera Rebuild Stabiliser.

This is a two-part epoxy primer – you simply mix the two parts together in the correct ratio, and brush it into the holes/gaps/cracks.  Use an old brush that you no longer care about, because you’re never going to get it clean afterwards. And be thorough – it’s better to apply too much of this stuff than not enough.

It hardens up any remaining soft areas of wood, prevents future rot, and (if you time it right) is still slightly tacky when you come to do the filling, which hopefully helps the filler to bond even better to the wood.

removing all rotted wood before making a repair

Step 5 – Filling

For minor repairs and cracks, an ordinary wood filler such as Toupret Wood Repair Filler, or a two-part filler such as HB42 Ultimate Wood Filler, will usually be fine.  For larger and/or deeper areas though, it’s better to use a two-part epoxy filler such as Oxera 4 Hour Rebuild Resin.

This is a two-part system – if you’re doing a lot of this type of work, it makes sense to buy the cartridge which automatically dispenses the two components in the right amounts for you to mix together.  On the other hand, if you only need this type of product occasionally, you can buy the two components in small tubs and mix together yourself in the correct ratio, which is the cheaper way to try the system out for the first time if you’re new to it.  The beauty of using something like rebuild resin is that it will never rot, it’s rock solid yet has enough “give” in it to expand and contract with the timber.  Whether you use the tubs or the cartridges, combine the two component parts together on a board until thoroughly mixed, and then use to fill as required.  It’s worth noting (before you start!) that these products are not easy to sand once they are hardened, so take your time to make your repairs as perfect as you can while you have the working time.

Oxera also do a 1 hour Rebuild Resin, but I prefer the 4-hour because it gives me longer to get the surface as smooth as I can.  Top tip:  once you have mixed the two parts together, spread the resultant resin out on your board as thinly as possible – this will give you a longer time to work with it.

It sounds counter-intuitive, but if you leave it mixed up in a big ball it will start generating heat from the reaction and will “go off” much quicker.  The bigger the surface area, the longer it will take to “go off” (also, a lower ambient temperature will help too).

a window repair made using oxera

Step 6 – Painting

Once all your filler is fully cured you can sand any ordinary filler (not the epoxy resin type though) to make it smooth, and then paint over with whatever your preferred paint system is – if you want some advice on this, click here to read about the best paints for exterior wood.

 

The Products I Recommend for Repairing Rotten Wood

 

The quality of your materials (along with the prep) ensures longevity of repair. There is no point in ‘cheeping out’, if it means you need to make the same repair again a year later. I’m going to briefly list the materials I use, explain why I use them, and what for.

 

Oxera Epoxy Primer

This is the wood hardener. Toupret make a good alternative, as do Ronseal. However, I feel Oxera is probably the best when repairing rotten wood. Simply mix the two parts together and apply a generous amount to the wood (careful it isn’t pooling or running).

Zinsser Peel Stop

This is used around the edges of flaky paint. It seeps into the timber and behind the remaining paint, sticking down the edges and preventing further paint peel. Epoxy primer also does this from what I can tell, but if you’re not using epoxy primer, get yourself some Peel Stop.

HB42 Ultimate Wood Filler

This is the best 2-part filler I’ve used. It doesn’t slump too much and it’s quite fine when you sand it. This filler doesn’t contain styrene, so it’s safer to use. This is perfect for small repairs.

Oxera Resin Repair

Repairs made using epoxy fillers will always last longer. It acts as a strong adhesive, and has movement, so expands and contracts with the timber. There are several brands available. Oxera is reasonably priced, and comes in pots, so you don’t necessarily need to spend money on expensive applicators.

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FAQs

 

Can I use wood filler on rotted wood?

There are different types of wood filler you can use when repairing rotten timber. Water-based fillers can be used for pin holes left by nails. 2-part fillers can be used for small repairs. Epoxy fillers for bigger repairs.

 

Can I fill rotted wood with concrete?

Concrete will not expand and contract with the timber, so if you use it, then your repair will not last.

 

Can you stop wood rot once it starts?

You can, but you must ensure you carry out the steps in this blog correctly and use good-quality materials.

 

Can you just paint over rotted wood?

You can paint over rotted wood, but it’s a pointless task. Your paint will fail and flake off in no time.

 

What is best to fill large gaps in wood?

Epoxy fillers are the only products that will last in a large repair. If the gap is particularly big, then you can pack it out with timber before you apply any filler.

Final Thoughts

 

Like a lot of decorating projects, repairing rotted wood is “all in the prep”.  As an example, on one recent external wood project, I spent six days on prep and only three days on painting.

But if you don’t get the prep right (removing all loose and flaky paint, digging out all the rotten wood, treating it to harden it and stop the rot returning, and then filling it with the right product), any time you spend on painting could ultimately be wasted.  The products we have available to us nowadays are so much better than they ever have been, which is a massive help, so make the most of them to do a thorough and professional job repairing rotted wood on your next project!

Updated Sep 3, 2024 | Posted Jul 30, 2024 | 1 comment

About the Author

About the Author

Mike Cupit has been in the decorating industry since 2002 and has mostly worked as a Trade Decorator in the domestic sector (peoples’ homes). Self-proclaimed “product geek”, Mike has a passion for paint and decorating tools. Mike now spends most of his time testing paint products and tools, comparing them to similar products on the market, and blogging about the industry in general.

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1 Comment

  1. Sheila

    Thank you for this helpful, and timely, article.

    The information here has confirmed many of the suspicions building up about our “professional” decorator’s practices. I thought the continuing rot and peeling paint on the original victorian sills was something we just had to accept. I thought it was normal for it to fail in a few weeks and require regular painting. But when a brand new sill rotted in less than 12 months, something was obviously up.

    Lesson learned. I’m going to follow your excellent advice and do it properly myself. I’ll also not be painting in early spring when the wood is still damp, when it’s raining, using water-based gloss over oil-based without first using a primer or fail to wash off the woodwork after a cursory sanding, where muck is still clearly visible. I was assured none of these things mattered either. I should have listened to my gut!

    Reply

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