paint a tounge-&-grouve ceiling?

I wouldn't have thought you'd need to prepare tongue-and-grooved wood, even if the knots were visible. As long as it's not greasy the emulsion should grip well. If there are any cracks or whatever round the knots that show la ter, rub it down with sandpaper and repaint the spot.

paint the knots with pva glue ... wait 24 hours ... then paint as you want

Hi We have a small wooden tounge-&-grouve ceiling in our porch with knots visable that has been varnished. I would like to paint it with white emulsion. What preparation would be needed? Thank you