In 2006 I rendered a house extension which had been built with artificial Cotswold stone, hard,slightly glazed, unattractive and unconvincing. So the owners thought it might be lime rendered.
I remember thinking that there could be a problem with adhesion given the hard nature of the structure, however, I devised a plan:
I brushed on a thick lime wash and whilst it was still wet I harled it with a lime course stuff, then I trowelled this coat flat, waited for it dry and then harled it again. After which I gave it 4 maybe 5 coats of limewash.
Today I went back.
The render is almost prestine although it has got a bit grimy where water has been running down the wall from gutter overspill, and some of the lime wash has washed off from the most exposed elevation, but overall its great.
Unfortunately, I cant remember weather it was hydraulic lime or simply lime putty.
But some observations:
The aggregate was Cotswold grit sand.
the render is tough, robust yet thin.
The render appears to be able to absorb some moisture and to let it go again even though it has no need to, because the walls certainly don’t.
so was it hydraulic lime ?
It has stuck well to the concrete substrate but in the wet you can just make out the cement joints beneath the render – It breaths, or appears to be able to.
was it lime putty?
Its hard. Its tough. It withstands the elements.
I should have kept a record I suppose but whatever lime I used it worked very well and looks great after 7 years.