Plasterboard joint?

If you use taper edge board, and then the tacky fibreglass tape, and a decent width filling knife (ready mixed filler makes it easier) same over screws, you might not make it good enough to avoid sanding, but use some 400-600 grit paper and it should be nearly invisible

OK sounds good, question though do I push the tape into the taper or just lay across it before using filler.

No - you lay it along the joint, half on each side lift taping a parcel shut. It's to bind the edges and to diffuse cracks.

Some Youtube videos show the technique

I've used this method with self adhesive fibre tape (rather than using 'mud' as the adhesive) and with minimal sanding afterwards

Even with straight edged boards, you can make an invisible joint, but you need to take a bit more time to build up the filler and taper it well out, and don't get carried away with the sanding.

just lay it across, it's lightly self-adhesive.

That's square-edge rather than taper though.

Its perhaps worth adding that the taper on the board is several inches wide - you typically use a 8 - 10" wide filling knife for filling the section where the tapers meet.

OK thanks John.

Yes, but a bathroom is the one place I wouldn't take that shortcut. You'll be left with a paper ceiling in a room where condensation is frequent.

NT

Won't the paint protect it?

no. It may get mouldy in time, some do some don't. When it does you cant remove it without removing most of the stregth of the pb. So at least mouldproof it.

NT

I've got an an un-skimmed plasterboard ceiling here in a bathroom, it's never had a problem, any more than the similar walls have.

If it's bad enough to have that much condensation lasting that long I'd suggest that the ventilation needs improving.

Paper supports mould growth, plaster doesn't (although the paint on the plaster probably will).

Making sure it's insulated behind the plasterboard will minimise condensation. I redid a lean-to bathroom ceiling and could only fit 25mm PIR board behind, but that has completely prevented condensation on the ceiling.

I just recently added a 6" extractor fan +timer +humidity, previous to that I had condensation on the walls/mirror (after showering) now it is totally clear, so ventilation has been improved.

If it really is a small area, tile it? That's what I did in my larger than normal home made shower cubical. Got some really cheap white wall tiles - they don't need to be strong like a wall one.