External re-pointing advice please

[vicky whitworth]

Hello, we have a 1750 rubble stone cottage (not listed). I have raked out all the old pointing and I now have a couple of questions.
1. The rubble stone walls are made up of quite a high percentage of chert (very glassy, hard surface a bit like flint). It has been painted in the past with something like sandtex over old limewash. A lot of the modern paint has chipped and flaked off as I removed the old pointing but it is adhering well in places. I have been told I need to get it all off prior to limewashing or the limewash won't stick to it. Is this true?
2. If it is true how should I get the paint off the chert? I have had no success with paint stripper in the past on the walls so someone suggested a needle gun. What do you think?
3. I have also been told that I need to rake out the old mortar to twice the depth of the gaps between the stones but this is really difficult on a very rough rubble stone wall eg: in places the gaps between stones is several inches across which would mean raking out to 4-6" deep. What would you do?
Many thanks, Vicky

 

[Jon Maine]

Vicky,

Yes best to remove the plastic paint.

I would always recommend a pointing depth of 20 to 25mm deep, if you have wider deeper holes push in a bit of broken tile to help stop shrinkage. Dampen joints down well first, also helps wash out the dust.

Needle guns are good but labour intensive and slow.

Alternatively as long as all your stone is hard you could look at shotblasting, this could also clean the joints out for you. It can be very aggressive and filthy so if you do consider this you need someone who has a light touch. Not a process fit for much work on historic properties but sounds like it may be right for yours.

Best of luck

Jon

 

[Penners]

What paint stripper have tried? Strippers of Sudbury (who make Klingstrip) have a useful web page advising which of their formulations to use for which materials. Have a look at this: http://www.stripperspaintremovers.com/I_want_to_remove.htm#11

You can buy small tester pots from them. I suggest you give them a call and ask them specifically about removing Sandtex from chert.

Personally I would be wary about using a needle gun. You could run a considerable risk of causing damage to the stonework. It could also take the rest of your life to do the job!

 

[chuckey]

Wrt raking out, it depends what you are expecting the repointing to do and the state of the original mortar. For the worst case, horizontal course, original mortar is broken up into loose clumps with a lot of dust. To me this means two things, that the mortar is not supporting any weight and has failed in its purpose to keep the weather out. In this case I would rake out to a depth of 4" and regard the "pointing" as a replacement mortar course. The other extreme where the mortar is extremely hard, here, new pointing is only required to get a colour match, so raking out to 5/8" should be enough to get a fillet of the new mortar in.
My rock and rubble walls have a notional inner and outer skin of say 6" and a wall thickness of 24". So I never excavate a depth of more then 1" unless the old mortar has failed(see above).
Vertical joints don't support the building and, in my case 90% of them have only loose mortar in them. I tried to fill them to a depth of 4". In many cases, while the gap in the front of the stones might be 1/2", because the sides of my stones tend to taper away from each other, the gap at the back might be 6". It is extremely tedious to try and shovel mortar through the narrow front gap to fill a gaping void at the back.
The other thing I do is to, what my local builder called "brutalising" the stones. Some times you have two rough edges of stones that touch somewhere, then open out into a pointable gap. I remove the "touching" bit by chipping the front edge of the stone, so a bead of mortar can be run right across the stones.
Frank

 

[FamilyWiggs]

Vicky,

I have often found the abrasive power of high pressure hose (of the mini-compressor type) to be quite surprising. If a lot of your Sandtex has flaked off, it might be worth trying one close up - it may get enough of the rest off to avoid shotblasting or expensive chemicals.