Re-skimming with lime laster after repair to wall

[Jelfy]

4 years ago i ripped out the nasty sand&cement render from my chalk cob and old brick walls (internal ground floor)
and had it redone with lime putty (classic 3 coat method) then painted with a silicate paint

Quite a large area of the wall then started to flake up and then the top coat failed and in patches some of the float coat too. The scratch coat seems sound. I cant fully work out why
I think it went on too thick and too quick see pictures note how the silicate paint is coming off too....



Last week i repaired with some NHL3.5 and sharp sand and now i want to reskim the whole wall with some premixed lime putty top coat, and paint in limewash



So my question is: will the new skim stick to the area of original wall covered in a thin coat of silicate paint or will i need some sort of primer like DG27 and would i put that over both old and newly plasterd areas?
Also any idea why the plaster failed....?

Thanks you
Chris

 

[GPHouse]

Looks interesting! I cant comment on why it failed - there are people here far more qualified than I to answer that question.... but....

I tried skimming on some old lime without a primer - I'll call it an experiment. even with a bit of a key the results weren't great. Areas where I invested in DG27 have been fine.

I waited for the newer patching (both full depth coarse lime/hair and lighter lime mix) I did to dry out pretty much totally before priming the lot. 12 months on it seems solid.

For the cost of a bucket or two of DG27 I would just grab a couple of 10L DG27 tubs and avoid the heartache....

 

[Chris-Burgess]

looks like you have negated the key on the scratch coat, which by the looks of it isn’t heavy enough anyway, Render mesh has to be inside the render not on it. Think of resi bar in concrete if the resi bar was on the floor and you poured concrete on top of it, it wouldn’t do anything, same as the mesh. If you wanted to use mesh it would have been better in the scratch coat. Coat of stuff on the wall, push mesh into wet plaster then coat on top and scratch. Diamond scratch as this covers more surface area and affords a better key between the scratch and floating coat. You must push plaster through the mesh so it can bind which can be difficult if the aggregate in the mortar is a larger grain, It does look like you’ve used the larger mesh which is right, but.If the render mesh is loose it’s all going to fail. No need to use primer on new plaster float coat just wet it down.

 

[a twig]

Putting hydraulic lime coats then fat lime over the top? Entirely wise?