Filler for hairline cracks in external timber

[Zebra]

The type of filler to use for substantial repairs to external timber prior to painting is a topic that has been covered a fair bit on here recently, and the consensus seems to be that a 2-part epoxy is best. Chemfix Timbabuild was one that was recommended.

For hairline cracks however, such as appear all over the sill, or such as appear at joints in the frame, what would be recommended? Where I've repainted without filling, the cracks have reappeared.

The paint to be used is Bedec MSP, or possibly Demidekk Ultimate.

 

[Toby Newell]

Epoxy is only used in the worst case scenerio, if you need to use epoxy in many instances you would be better off letting in a bit of wood.

A normal 2 part polyester would be used for smaller cracks of which there are many dozens to choose from. In fact any 2 part polyester will do from U POL, Isopon etc through to Ronseal High Performance, car body and wood finishes are mostly identical.

There is no guarantee the cracks will not open if the wood moves there.

Modern SMP polymer (MS Polymer / Hybrid / Modified Silane) adhesive - sealants, of which there are now hundreds to choose from might be better in high movement areas as these normally have 20%+ elasticity. They are moisture curing, water resistant (some waterproof) and mostly overpaintable. Which might be better for high movement areas.

I would say that they are not really sandable, so only good for small gaps where you can apply and smooth off with a wet finger to a finish.

PS remember to scrape and clean out your cracks before you refill.

 

[Zebra]

Thank you Toby

 

[Toby Newell]

something like this, there are many now.

 

[Feltwell]

If you do use CT1, buy an aerosol of Multisolve as well - you'll need it - CT1 is great but very sticky, you need Multisolve to smooth it. Personally, I'd stick to something I could sand for minor cracks in wood, though as said above the CT1 will allow far more movement.

 

[how999]

Putty and linseed paint. Nothing else allows the wood to breath properly

 

[malcolm]

I use linseed putty in that sort of gap too. I'm a fan of linseed paint too because it doesn't have to be stripped off like that after only a few years as it doesn't crack.

 

[Zebra]

Putty and linseed paint. Nothing else allows the wood to breath properly

That's what I'm removing as it's flaking.

 

[malcolm]

That's what I'm removing as it's flaking.

While it has it's faults, pure linseed oil paint won't crack or flake unless it was painted on top of some other paint that that does crack. Do you mean the filler was flaking?

For joints like that linseed putty is quick to apply and seems effective to me. I do have a joint in a window sill repair that expands and contracts with the seasons and pushes the putty out so maybe not the best choice for that. Though I wouldn't like something non-breathable in a joint as anything you put in is likely to crack and let a little water in.

 

[Toby Newell]

Linseed putty is good for very small cracks but takes months to dry and is hard to coat with anything other than oil based paint. It also, eventually dries very hard, shrinks and becomes brittle, so worse than an SMP polymer, breathability has almost 0% to do with durability in this case.

It's not so good for larger areas. It does harden, breathability is a bit moot, shrinkage and elasticity are more important properties in a filler.

Linseed oil is quite a low quality material that was only used as it was originally a free by-product of the linen industry.

I actually think putty is a great material in it's usefullness and simplicity.

The main problems with linseed oil products are curing times which range from days to months, incompatability issues, susceptibility to mildew (can be mitigated) and shrinkage, also oxidation makes light colours go yellow. Boiled / stand oil dries faster but is weaker and goes more brittle.

It's always good to use a range of materials both traditional and modern to try and obtain the best of both and not be beholden to any.
I am sure putty is great for some areas, peroxide cured polyester, two part expoxy and SMP polymer better for others.

As a side note I will repeat 'breathability' is a marketing term sold to Joe public who aren't scientifically literate.

EVERYTHING is somewhat breathable and NOTHING is completely breathable, just more or less. Film thickness is not often mentioned. A higher performance finish with lower film thickness will be more breathable than a lower performance finish that requires a thicker film. Permeance vs Permeability, its the performance of the system that is important ie the combined thickness of coating + substrate.

Highly pigmented linseed oil paint in a 2 or 3 coat system is not necessarily any more breathable than a more highly engineered paint, that's a scientific fact, not received wisdom. Some PVA / PVC and mofified acrylic and acrylic styrene polymers have better Permeance performance than thicker linseed oil paint systems.

Green marketing waffle is promulgated and persisted by people's reluctance to research, so fair play to them. The human brain is incredibly susceptible to reductive reasoning, ie plastic is pretty waterproof, oil finishes are 'natural' which is very persuading but facile. Cured linseed oil forms a thick, inflexible plastic film that can be less permeable than an acrylic one (without research I suspect Bedec et al are acrylic based)

Easy to understand example:

I guess most people reading this would assume that a Coke bottle is pretty waterproof, seems intuitive.

In Tescos pick up a plastic coke bottle (or ketchup) and a glass one and check the best before dates....

Vapour permeability of PET means oxygen can get in and hence the BB date will be months or years less.

I am not saying linseed oil paint is good or bad, it has it's place.

I AM saying it is a cheaply repeated myth (like the guy who said wallpaper paste is now all made of plastic because someone told him so) that is patently, scientifically untrue.

An alternate way to (incorrectly) use reductive reasoning is industrial use. There are millions of sqm of modern new build buildings coated every year, the timber of which is often best served (depending upon the climate!) by using a high permeance system. The vast majority of which are not linseed oil based. Just a thought.

I have used hundreds of gallons of shellac and I love it! However, it's not used in commercial buildings. When a customer wants a high performance finish I use industrial, commercial ones you cannot buy in Wickes. I love shellac and it does things modern finishes cannot, it does not mean modern finishes are always bad.

I think the summary of todays lesson is that linseed oil products, especially putty and paints, are traditional, quaint and have both good and bad properties. But, to state they have better properties than all modern finishes is factually incorrect.

Final comparison:

I have never used a single sheridised twisted square nail in my life despite my known penchant for fitting thousands of noggins variously around the country. A few years ago any carpenter / builder / framer worth his salt would scoff at my, possibly dangerous ignorance. What they didn't know was advances in manufacturing of screws meant that modern ones were not only stronger than nails, have better retention but now are less brittle. Proven by the fact that world leader Simpson Strong-Tie now provide the highest tensile data using their own torx screws, like I have been using for years.

Any builders / framers / roofers happy to continue to give themselves RSI using twisted nails, great, twisted nails look beautiful and do a great job. But they are not better than modern, high performance screws.

As usuall, carry on.

 

[Zebra]

While it has it's faults, pure linseed oil paint won't crack or flake unless it was painted on top of some other paint that that does crack. Do you mean the filler was flaking?

For joints like that linseed putty is quick to apply and seems effective to me. I do have a joint in a window sill repair that expands and contracts with the seasons and pushes the putty out so maybe not the best choice for that. Though I wouldn't like something non-breathable in a joint as anything you put in is likely to crack and let a little water in.

It was Holkhams linseed oil paint, it was applied on brand new windows, with linseed oil applied first, and where it's in direct sun it has flaked. I've patch repaired it every year and am fed up of doing so......

 

[malcolm]

It was Holkhams linseed oil paint, it was applied on brand new windows, with linseed oil applied first, and where it's in direct sun it has flaked. I've patch repaired it every year and am fed up of doing so......

I didn't think Linseed paint could crack but sure enough I've taken a look at my south facing window and the white linseed paint is peeling off. The black to the west doesn't peel but dusts instead.

 

[mardinor]

My experience with similar is that normal 2-part 'car body filler' (UPOL or Ronseal equivalent) cracks after a couple of seasons, causing failure.
I use RepairCare Dry flex SF (surface finish). This is a specialised product for this application and is flexible. It is for 0-6mm cracks, whereas the usual dryflex only works for gaps over 6mm.

 

[Cubist]

Rightly or wrongly I've never believed the problem lies with either the paint or any filler that may have been used in the joints of window and door frames. The culprit, I believe, is the differential movement between the vertical and horizontal timbers. Any filler, except one that precisely, or very closely, matches the rates at which the surrounding timbers may expand and contract, will inevitably be repeatedly compressed and stretched by the wood as it shifts and ultimately it will disintegrate. Even if a filler could match the typical rate at which a given wood expands and contracts then it would still have to cope with the fact that wood expands and contracts at a greater rate across the grain than it does along its length.

To deal with the problem I have simply pressed caulking cotton, where possible, into the seams as this at least has some ability to continuously expand and contract as it gets damp and dries. It can then be painted with a little linseed oil and overpainted with whatever final finish/colour one wants. Sadly, even this approach ultimately fails and cracks will again begin to open up if the paint itself cannot handle the rate of movement. However, it will help a little to prevent water penetration into the frame which in turn may slow the rate of movement in the key areas.

 

[Toby Newell]

The culprit, I believe, is the differential movement between the vertical and horizontal timbers. Any filler, except one that precisely, or very closely, matches the rates at which the surrounding timbers may expand and contract, will inevitably be repeatedly compressed and stretched by the wood as it shifts and ultimately it will disintegrate.

Yes!

 

[Toby Newell]

My experience with similar is that normal 2-part 'car body filler' (UPOL or Ronseal equivalent) cracks after a couple of seasons, causing failure.
I use RepairCare Dry flex SF (surface finish). This is a specialised product for this application and is flexible. It is for 0-6mm cracks, whereas the usual dryflex only works for gaps over 6mm.

Very interesting, has the word flex in the name yet zero information on the TDS or anywhere regarding the modulus of elasticity or maximum elongation at failure, basically, its flexible, trust me bro...

It might be perfect for exterior repairs. I have reccomended it as being the strongest, most waterproof filler for deep holes but always, perhaps wrongly, assumed it was not that flexible due to it being so hard.

Still incredibly strange that a product known for being incredibly hard and inflexible but yet has 'flex' in its name as its USP has zero statistical data showing it's 'flex' capabilities, you know, when many order of magnitude cheaper bathroom sealants do...

I must investigate! I always thought, referring to Malcolms' excellent comment, that the durability of the epoxy finishes was in fact the fact that their strength and bonding power was so great that they literally resist movement rather than allow it and that any failure would be transferred to the wood itself. (cf. hard cement render between soft stone brick causing spalling)

Cured SMP polymers remain as flexible as the sole of a trainer shoe whereas epoxy is about as stiff as Perspex (PMMA)

No idea why they do not publish elongation and modulus data.

I guess the hallmark of a great adhesive / filler is flexibility as much as bonding power and tensile strength as EVERYTHING moves.

FYI check out 3C sealants, they do excellent products and are very knowledgeable, their comparative product is around half the price of the one you mentioned.

EDIT:

3C have just launched what looks like a comparable product which is a third cheaper on promotion.

 

[wobs]

It was Holkhams linseed oil paint, it was applied on brand new windows, with linseed oil applied first, and where it's in direct sun it has flaked. I've patch repaired it every year and am fed up of doing so......

I used Holkhams on my last house, about 13 years ago, and the front is south facing. I went past last Friday, and it still looks good, even though it almost certainly has had no maintenance.

The bay is a hardwood, and was new in 2003, I stripped the gloss back to bare wood. The front door is probably about 50 years old, and I stripped it back to bare wood.

 

[mardinor]

Very interesting, has the word flex in the name yet zero information on the TDS or anywhere regarding the modulus of elasticity or maximum elongation at failure, basically, its flexible, trust me bro...

Fair enough. Agree it's strange they don't show data.
All I can offer is empirical evidence. I've used both the deep repair and this SF surface crack version on 12 south-facing 120 year old sash windows, beneath Jotun Demidekk paint. They have shown no cracking 10 years later. But this is just one data point.

 

[Toby Newell]

Fair enough. Agree it's strange they don't show data.
All I can offer is empirical evidence. I've used both the deep repair and this SF surface crack version on 12 south-facing 120 year old sash windows, beneath Jotun Demidekk paint. They have shown no cracking 10 years later. But this is just one data point.

Yes, very strange, I might ask 3C, they have just come out with a similar product, they normally have great technical data. No, they look good, moot for me as too expensive to use as a mass gap filler for floors, but if I ever need an exterior filler I will certainly try them out. The epoxies are some of the highest performing coatings and adhesives there are, so they are probably very good. I mean, they make rockets and fighter jets out of the stuff, I guess window cills are within their performance envelope.