We have painted many period homes over the past 28 years, and it is always of the utmost importance to us to try to keep as much detail as possible in the plasterwork when redecorating.
Sometimes, over many years, often times without the same consideration, the paint accumulation covers the detail and the only solution is to strip the cornicing. This is a painstaking and costly business which involves stripping solutions and dental picks!
So when we come across a cornice which has to be painted, we will generally spray paint it as, when done properly, it will not fill in the detail too much.
If it is a case where it has not been painted in many years we would use a thin oil-based, stain-blocking paint to whiten the surface without having to use many coats of paint, and then we finish with a spray emulsion.