skimming over blown plaster

Hi all, I've got a small bathroom ceiling to replaster. There are 2 problems:

1) The old skim has blown in a few places, but the ceiling itself seems stable (looks like concrete),the blown plaster patches are about 2mm deep, so not deep.
2) The ceiling was never painted but instead given a clear glossy varnish (the client reckons it was unibond, and she wants it reskimmed and varnished again so it 'looks like' venetian plaster)

I'm a decorator with some experience of plastering, but not with a ceiling like this. My question is once I've scraped the blown plaster and pva'd it twice, can I reskim the whole thing in one go or do I have to fill the patches up first with bonding and skim next day?

the 2nd question is what do I seal it with bearing in mind it's a bathroom? Just a waterproof PVA? Zinsser have this clear sealent called Gardz, wondering if anyone's familiar with it? Any advice will be welcome.

It depends on what thickness needs re filling in the plaster coat, I would scrape off all that comes of,gyp bond or similar, if its a bit rough put two handfuls of bonding in the first coat,flatten,let it pull in then skim with a nice skim coat,

Scud it

The thickness is around 2-3mm. I'm hoping to do this in one day, ie., to scrape off loose stuff, PVA it and reskim it, then seal the whole thing a week later. I asked a plasterer about it the other day and he thinks it should be ok....so here goes!