Best Wood Filler for Rotten Sills: 2025 Expert Review
DR Joinery
Expert Joiner & Restoration Specialist
Rotten sills are commonplace in York, where our damp weather and older housing stock create the perfect storm for timber decay. Many homeowners make the mistake of digging out a bit of rot and slapping standard filler over the top, only to find it cracking and falling out after the first frost.
True restoration isn't just about filling a hole; it's about stabilising the timber and creating a repair that moves with the wood. In historic joinery, we often prefer splicing in new Accoya or hardwood, but for smaller areas where replacement isn't necessary, the choice of filler is critical.
In this review, I'll walk you through three products I use on site: a budget option for surface finishing, a heavy-duty two-part system for structural repairs, and an essential hardener that you must use before filling anything.
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Specs vs. The Real World
The Spec: "Fills any depth"
"Manufacturers often claim one tub can fill deep holes in a single application."
Ready-mixed fillers sink and crack if applied deeper than 5mm. For deep rot, you must use a two-part epoxy or polyester filler that chemically cures, or build up in layers.
The Spec: "Stop Rot Formula"
"Some fillers claim to cure the rot as they fill."
You cannot fill over wet, soft rot. It will continue to decay underneath. You must cut back to sound timber or use a high-quality wood hardener first.
My Top 3 Recommendations
Three options to cover different needs and budgets
Bartoline Ready Mixed Wood Filler White 500g
Bartoline
This is a classic, straightforward filler. Whilst I wouldn't use it to rebuild a structural corner of a rotten sill, it is excellent for the final 'skim' coat over a repair to get a smooth surface for painting. It's affordable and easy to sand.
The Good
- Very affordable for general maintenance
- Easy to sand down to a smooth finish
- Ready to use straight from the tub
The Bad
- Not suitable for deep structural repairs on sills
- Will shrink if applied too thickly in one go
Installer Tip:
Don't try to fill a large deep hole in one go with this. Use it for the final 2-3mm to get a perfect finish before painting your sash windows. It sands much easier than the heavy-duty two-part fillers.
Priced at £2.99 at the time of writing
Check Price on AmazonRonseal High Performance Wood Filler, White, 3.7 kg & Hardener
Ronseal
This is the heavy hitter. When you have significant rot on a sill and need to rebuild an edge or fill a deep void, you need a two-part filler like this. It sets rock hard, doesn't shrink, and can be drilled or screwed into once cured.
The Good
- Excellent structural strength for rebuilding corners
- Does not shrink or crack (chemically cures)
- Can be drilled, screwed, and planed
The Bad
- Harder to sand than ready-mixed options
- Sets quickly - you need to work fast
Installer Tip:
Mix small batches on a scrap board. You only have a few minutes of working time before it goes off. Shape it slightly proud of the surface, then use a rasp or heavy-grit sandpaper to level it before it cures completely rock hard.
Priced at £41.98 at the time of writing
Check Price on AmazonRustins Wood Hardener for Rotted Wood 500ml
RUSTINS
This isn't a filler, but it is the most critical step in fixing a rotten sill. If the wood is 'spongy', filler will just fall out. This liquid penetrates the timber, hardening the soft fibres into a solid base that filler can actually stick to.
The Good
- Transforms soft, rotten wood into a solid base
- Penetrates deep into the fibres
- Prevents the rot from spreading further
The Bad
- Potent smell - use in well-ventilated areas
- Requires complete drying before filling
Installer Tip:
Remove all the loose, crumbling wood first. Then, drill small 3mm holes into the surrounding 'soft' timber to help this fluid penetrate deep inside. Apply liberally until the wood stops drinking it in. Once dry (give it 24 hours in our damp York climate), the wood will be solid enough to fill.
Priced at £12.48 at the time of writing
Check Price on AmazonFrequently Asked Questions
Can I just fill over rotten wood on a sill?
Two-part vs Ready-mixed filler: which is better for sills?
How do I get a smooth finish on a repaired sill?
When is a sill too rotten to fill?
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