Bedding in a wooden window sill and reliable, long-lasting window seals for wooden frames.

I am looking for an alternative to foam / silicone for bedding in a wooden window sill on to a stone sill.  The main criteria are for it to resist water & be long-lasting 

I don't find one, I will probably just use plastic window sealant in a gun.

Of course the best thing is to stop water getting in in the first place through good paint/seals but I would also like a bedding in product that is going to help.  This is the second time in twelve years I have had to replace these sills and there have been repairs in between times.  

The amount of weathering is partly because the windows are four stories up, facing south, in Scotland (and I could have been better with my maintenance).

The sill is going to sit on stone and back up against the wood of the frame - or rather, against the two part filler I have used to protect the frame at the bottom.  This filler at the front of the frame has yet to be sanded but you can see the idea.  

The frame is partly wrapped in tape only to stop it sticking to the filler when I test it for fit.

There is still a gap though, between the frame and the sill...

I used foam on the first sill I installed but the two-part wood filler I was using to make a seal for the front of the windowsill did not adhere well to the foam, which itself is very friable.  

The two-part tended to crack when applied to the foam just as a thin seal.

 I had to dig out at least good inch of the foam from under the sill in order to make the wood filler seal deeper.  It is not easy to smear in the filler either, into such a small gap, never mind leaning out the window four floors up.  It is a time consuming and smelly job! 

UPDATE:  A friend who has replaced window sills on an old property said the traditional method is to use batons under the sill to keep it off the stone.  The idea is for there to be air under the sill.  In the event that water does get in, the sill is not sitting in water, rotting.  The batons may rot, but can be more easily replaced than the sill. I am quite taken with this idea.  I only have a finger's width between my sill and the stone though so any batons would have to be slender.  

I would find the idea that rain might get in past the seal or down the sides of the frame (both classic water-ingress points)and head straight for the wooden frame at the back of the sill, quite terrifying, given what it can lead to. But, given how well I have sealed that with two part filler, I am not so worried. 

My friend also said condensation inside the room can also cause damp.  I said that back we had money to pay people to do things like this, I ensured inspection hatches were put in the plasterboard on the window-wall side.  We keep these open to keep an eye on things and allow air circulation.

 

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Two part wood filler may seem unusual as a sill seal.  I am considering going over it, belt and braces style with silicone/acrylic window sealant.  

I have also resorted to using it as such on the vulnerable lower sections of the external sash frame. You can see it (the yellow stuff) on the right hand side here.  In this picture it still need to be sanded / painted.

The as yet unpainted block is just to protect the sash frame at a commonly vulnerable point. More on that here. 

When excavating the rotten windowsills the one part that was untouched by rot was a repair that had been done with two part filler.   So I know that water just doesn't penetrate. It was so solid we shaped this and fit the windowsill to it rather than dig it out.

So I'm pretty confident it'll work as an outstanding, if expensive & tricky to apply, seal for the sill and the most vulnerable parts of the exterior frame.  Well worth the money.  I will only use it on the lowest, most vulnerable sections of the frame as shown above.  It can be sanded and painted over.

In the past I've seen silicone round the frames.   It doesn't last and peels off in stringy strips.  I've made up a traditional kiln dried sand / linseed oil paste called burnt sand mastic which can be very good but which can also crack. It's also quite time consuming to make / apply.  An example in the lower section here:

You need to get the sand from a reputable supplier as it needs to be totally dry.  Some of the stuff I have dug out below rotten sills has been quite damp.  I'm not sure if that's from water getting inside the sill from the sides though.  

The most durable thing I've seen on the windows is this:

  Unfortunately, I'm not sure what it is...