Butting Lime against Cement?

[TVW]

I plan on lime plastering a section of wall that was heavily cement rendered, inc a gypsum skim. Externally, there is still cement render (which will be removed in time). It’s been this way since the 1980s/1990s minimum.

My logic being that as the area has been suffering from damp - some lime is better than none to help breathability. I intend to do the whole wall (inside and out) but, there are more pressing things to get on with. Therefore, I am planning on butting up to the cement render (as pictured), lime rendering over the exposed brick and stone (now I’ve patched it up).

Questions:
Do I need to prime the cement edge?
Do I need to prime the stone or brick?
I assume haired coat to start is best?

FYI the ‘red’ brick nearer the base is fairly black. I don’t think this is soot but, maybe oxide ‘rust’ from the damp environment?

Any thoughts or help appreciated- very much appreciated novice, first ‘lime’ house.

 

[elizabeth]

Hi TVW, is that a line of bricks for a fire place halfway up? The black on the bottom is either ‘ black jack’ bitumen where someone has tried to seal the area from the damp or it’s mold, in which case you can wash it off with water & disinfectant/bleach but it may return through the lime for a while till it dries out over time. You shouldn’t need to prime anything, just make sure you spray/dampen down the area before hand to get the suction for the lime, haired lime is good for reducing cracking, take it up to the edge of the gypsum and then do a fine coat after the first coat has greened up. Then limewash over. We did the same, it was obvious we had two different materials if we looked closely but it blended in enough to live with for a while. I’m sure someone else will add advice or correct with more knowledge than me but that’s what worked for us. Good luck