Painting Mistake

[AKMild]

I'll be the first to admit that I'm not a great DIY-er, and true to form I have come a cropper on a very small painting job. It's a shower-room ceiling that I removed the existing flaking paintwork from and filled/sanded. I then wanted to size the filler/bare plaster with diluted emulsion as a prelude to painting. I found some white undercoat in the shed and proceeded to dilute it 50/50 and painted the ceiling with it. When I came to clean the brush I was surprised that the paint did not come off with warm water, and it turns out that I have used a solvent-based undercoat rather than an emulsion! Own goal!

Could someone advise me what I need to do from here to get the ceiling ready to take undercoat/topcoat of emulsion? I imagine it will involve letting the 'wrong' undercoat dry and then sand it off before sizing with dilute emulsion, but some advice from those in the know would be welcome.

[melv16]

Emulsion it as normal. Before the invention of stain block, oil based undercoat was used.

[Khandro]

Emulsion is OK on oil, it's the other way round is the problem, but in any case, just try a small area, - a couple of inches - overnight to see how it dries.

[AKMild]

Thank you both for your replies. I am much relieved.