Sand mastic around sash case

[endecotp]

Dear experts,

I am repainting a sash-and-case window in a stone wall in a very old building.

I believe that the sealant between the case and the stonework is sand mastic, and as you can see in these photos it is no longer making a good seal:

Image

• endecotp
• 24 Apr 2017

Sand mastic gap

Image

• endecotp
• 24 Apr 2017

Sand mastic gap

What do you suggest that I do? Options probably include:

- Remove the case from the wall and refit with new sand mastic;
- Leave the case in place, and scrape out and replace the sand mastic;
- Fill the gaps with sand mastic;
- Ignore it!

Any suggestions?

Also - as you can see, the inside of the case is partially unpainted. I know that the sides of the sashes are supposed to be left unpainted, but is this also true of the corresponding parts of the inside of the case?

 

[KenGMac]

endecotp, good evening.

Suggest you do NOT remove the original old case, it will not go bask as it is now.

I would scrape out the old Sand Mastic and replace it with a suitable equivalent Sand Mastic.

As for painting the un-painted areas of the sash or the case, I would not, because timber will slide easily on timber [assisted greatly by the application of candle wax ??] if any of the sliding areas of the Sash or Case are painted then the paint will over time become sticky and the window will jamb up.

Food for consideration?

Ken.

 

[endecotp]

Hi Ken,

I think I could probably remove and re fit from the inside without much trouble as I currently have the wall linings removed, and before I discovered that those linings were the only things holding the window in place it almost fell inwards.... But I will try removing the existing mastic with the case in situ and see what happens.

Re the paint, this particular window is newer than most of the others and the older ones do seem to be painted down the insides of the case. Or at least I thought they were; now I look more closely I think perhaps they have just been primed but not glossed, or something. I guess I will leave them as they are.

 

[endecotp]

(Does sand mastic stick to old sand mastic? I think it's a sort of putty material, isn't it? So I guess it should stick? Can I soften it up somehow?)

 

[KenGMac]

endecotp, hi again.

I think I could probably remove and re fit from the inside without much trouble as I currently have the wall linings removed, and before I discovered that those linings were the only things holding the window in place it almost fell inwards.... But I will try removing the existing mastic with the case in situ and see what happens.

yes I know what you are getting at, BUT?? the Case was being held in place, not by any Stainless steel fixings, but by the internal "Lining" [could be timber, or simply plaster, externally the Sand Mastic completed the job of securing the Case in place, OK sounds hairy, but? there are hundreds of thousands of these windows held in by timber wedges, Plaster, internal timber, and external Sand Mastic.

" Does sand mastic stick to old sand mastic? I think it's a sort of putty material, isn't it? So I guess it should stick? Can I soften it up somehow?"

Sand mastic has bee around for a couple of Hundred Years, it is a mix of burned sand and double boiled linseed Oil, [suggest you Google this material] when mixed this material would stick firmly to most things, including you?? As for the need to remove all the old Sand Mastic? you will possibly find, depending on the age of the mastic in place now [and it looks fairly shot] it should come off in shards, the re-application of your preferred colour of Sand Mastic will be reasonably easy, and as above the new application sticks to everything! as an aside.

1/ the finished surface "should" be between 20.mm. and 25.mm. over the flat diagonal surface.

2/. Do NOT paint the Sand Mastic.

3/. Something not generally found? birds love to peck at the newly applied Sand Mastic, it is the "Aroma" of the linseed Oil.

Hope this assists?

Ken

 

[endecotp]

I've found these notes about use of sand mastic:

http://hosting.lincolnshirelime.co.uk/PDFs/SAND MASTIC PDF.PDF.pdf

It talks about using a "mastic box". I was going to ask if anyone knew what a "mastic box" is, but then I found this:

http://www.masonsmortar.co.uk/knowledge/page/burnt-sand-mastic

which contains the same text plus a useful "terminology" section :

Mastic Box: Open ended three sided wooden box with handle to hold mastic while working 150mm x150mm x 50mm deep (a harling trowel can also be used)​

This all sounds a bit complicated!

 

[KenGMac]

endecotp, good evening again.

Must say the link you posted "masonsmortar" is in effect a definitive answer in itself.

As for the mastic box?? I think because the Mastic is shall I call it very tacky, and stiff, the box would allow you to form and pick up a well shaped length of mastic to apply, also using something like a "box" will allow the mixed mastic to be worked, made more pliable to allow the material to be picked up and applied.

Hope the above makes sense?

The other mention on that site is about using Masking tape, using the tape will Pre-determine the width of your finished mastic, and prevent the mastic smearing on to the frame and the masonry, also masking tape will allow you to tool the mastic smooth, as above the material is tacky and requires tooling to achieve a flat smooth finish. One thing, remove the tape as soon as possible followed by a final smoothing off with a suitable pointing tool.

Using this traditional material that has been used for hundreds of years and this type of Mastic has a very long life, is an easily manipulated material, but very tacky, and spreads, hence use the masking tape.

Just go for it.

Ken.