I have an old house,(1865), an I want to paint the bathroom ceiling. The ceiling is plaster in pretty good shape except for the hairline cracks, the old paint is sticking not peeling and I just want to freshen up the look.do I try to skimcoat the cracks or just paint over it? Anybody have any experience here? I don't want to damage the ceiling just freshen up the look.

Since you said "hair line" cracks, I would fill with a paintable, flexible caulking (such a bath and tile caulking). Since caulking shrinks as it dries, I would fill, scrap off flush with the plaster, let dry and do it again.

There should be no problem skimming your ceiling before painting. It has held up for the last 145 years, it should hold up a little longer.

Use Dap lightweight spackle to fill crack. Force into crack, let dry and sand. Dap does not shrink.

If ceiling is painted with semigloss or gloss paint, rough up surface with 200 grit sand paper.

Prime and paint.

Chuck

The cracks are all over the ceiling so fixing one at a time in not possible..
I tried to add pix but I am missing a security cookie? I have them but can't upload.
Like I said, the ceiling is in good shape so I guess skimcoat is the way to go

If none of the skim coat options work, consider overlaying 1/4" drywall.

If the whole ceiling has these minor cracks, it would be best to follow Brian's advice in post#6 and cover with sheetrock. There is a very good possibility that paint on ceiling has lead in it. If u cover with sheetrock, this will encapsulate the lead.

Chuck

Use a red masonry sponge float to work in and smooth over a paintable caulk. Double check, once caulk has cured, to make sure cracks are filled, add more if not. Light sand if needed then paint. This system is not recommended unless you're going to paint over it.