OK - now I understand - we are talking plasterer to plasterer - but you need some help with someone who has done 'Do Overs", escpecially in lime plaster. Luckily, I do have some experience in this area, on 1910 and 1913 walls.If the wall was not an existing three coat plaster wall, you have to make sure it can handle the weight of the new (10 to 15 lbs a sq ft - 4foot by 8 foot = 400 lbs). If it cannot - which is probably the case in this day of wood studs and drywall renovations/"improvements", then your options are limited.
If it is existing plaster, you must use like a light hatchet/axe to 'hack back' to a rough finish. You can tell from the sound of the plaster whether it is strong - if it is not, these areas of the scratchcoat will have to be repaired with wire and scratchcoat. Then it is a simple matter of plumb bobbing your straight vertical lines, you build up like a 'train track' for the darby to follow. More on this technique if you need it. The then final trowel coat, which is thin and consistent, so it is as flat as your second coat. The polish with the trowel at just the right stage and it begins to shine. It feels like cool, sensuous skin and the lime smells wonderful.
Over drywall, cement blocks, etc., is much more of a problem as you already know. It is difficult without assessing each situation - and you can still be wrong. If the cheap ass clients won't go for rebuilding the wall or ceiling, the only thing left is a skim coat. But you see you don't have your handy little 'train tracks' here - and there are probably 1 in 100 trowel workers who could make perfect flat. You can't see it for a start. You can use concrete bonder to help the plaster adhere. You can use a plastic 1/4 inch mesh too, from the stucco store, in areas likely to be cracked with vibration. Both these techniques require a thicker skim coat, and it is difficult to trowel without the latex bonder 'polluting' the lime surface. You will know right away when you hit latex -its almost like being on rollerblades, and then hitting a patch of chewing gum (not that you could get me on rollerblades!). So the practical answer to make a happy client with skim coat is to make sure it is colorful and flat luster finish - both these elements distract the eye.
If you are your own client - this is the chance to learn from outside the theory. Believe me - it is worth the effort. A perfect wall, in the right home - a pure white lime putty finish with glittery white carrerra marble dust worked into the final stage, then sealed with potassium silicate one year later - every time you entered a room such as this you would smile.
