Our rear garden wall is a mess and I’m looking for tips on what to remedy it with.

Built in a hurry to satisfy a neighbour, our engineer ruled that it could only be 1 leaf (wythe?) thick to avoid an overturning risk on the retaining concrete below. As a result the builder shuttered our side, so the neighbour has a nicely pointed stone face, but we get all the cement snots and occasional cavities. You can see from the photo there are occasional deeper piers at centres, so the length of the wall is broken up into bays.

Rebuilding is out of the question. So I’m thinking the best option would be a rough cast lime render to give some frost protection and hide the worst bits on our side. I’ve done some lime re-pointing on historic walls before, but never anything like this.

So:

1. Is a lime coat even possible here? The wall’s entirely built of nasty cement but I figure you can cover impermeable cement with breathable lime, just not the other way around, right?

2. What lime product/mix is appropriate in this case? I’d prefer something ready mixed due to my limited experience and lack of a decent mixer. Is a 20mm coat about right or nowhere near enough? I'm not looking for a flush/straight finish - if anything I'd like to leave some stones exposed but I'm now resigned to the fact that it's not good enough stone to leave on show, so we'll just have a whitewashed rubble wall finish.

3. I feel like I should try to gouge out some of the excess cement in the joints and repoint flush in lime, but it’s rock hard - is this advisable? What tools could I hire for this? SDS drill or something more like a mini-jackhammer?

4. My wife has pointed out that even if we achieve this, there's currently no coping at the top due to the wall thickness varying with the stone content. Are there any clever details that could stop the next heavy rainfall washing away my efforts?

I second what Tony said. I'd suggest using a (feebly?) hydraulic lime so you don't have to wait so long for it to go hard. I think coping stones on the top are definitely a good idea. You might want to try scratching any large flat areas of cement to give a bit better key for the lime.

If you go for only a 20 mm covering, you'll need to follow the surface fairly closely. It might be easier to flatten it a bit first as Tony suggests, though that will use more material.

I sometimes timber clad very rough walls - fix vertical battens to the wall with concrete screws at approx 600mm centres, then horizontal slating battens (25x50), with 8mm shaddow gaps.

Paint the battens with a watered down grey wood stain to kill the green of the treated slating battens, and give them an immediate weathered look. Super cheap, and visually very effective. Would show off the stone piers.

Oh, and paint something black over the stone first, so it doesn't show through the batten gaps. Definitely capping to the wall, and exactly as Tony suggested, paving slabs cut to size.

Just a thought

I rather like green paddy's suggestion must remember that one, you could use the battens as a trellis to grow plants up make a good feature in the garden and very green, economical and quick (apart from the plants growing that is)

Posted By: Doubting_Thomasdjh, that seems to contradict the advice here:
https://www.lime.org.uk/community/types-of-lime/types-of-lime.html

"Feebly hydraulic lime (NHL 2) – contains less than 12% clay which means it is slower to set (about 20 days in water);
Moderately hydraulic lime (NHL3.5) – contains 12%-18% clay which gives a slightly faster set (15-20 days in water);
Eminently hydraulic lime (NHL5) – has up to 25% clay content which means that a much faster set is achieved (hours/days)."

So am I after Eminently hydraulic (NHL5)?

If you want to use lime I think you basically want to use the weakest hydraulic lime you can get away with. Whether that is 'feebly' or not I don't know. Air/fat lime takes longest and needs reasonable control of moisture and temperature but is most flexible, most breathable and workable for longest. Eminently is strongest, most rigid, sets quickest and doesn't mind at all about moisture/rain. Ask your chosen supplier for advice about the best product to choose.

Posted By: Doubting_Thomas

What is the purpose of using lime here?

Simply that I'd rather not use any more cement than I have to, and all the advice for stone walls is that cement render coats are a nightmare in the long term. We're stuck with the cement 'core' that we have, but surely adding a slightly more flexible & breathable render on the outside would be better than whacking even more cement on top...

The advice to use lime base render on stone walls assumes that there is either lime or rubble/earth mortar in the build up of the wall. Given that you have a cement mortar which essentially amounts to a concrete with very large aggregate I can't see the problem with a cement render. OK cement doesn't breath like lime but with a cement/stone wall there isn't going to be much breathing of the wall with or without the render (of any sort).