I am just attempting my first job at coving (polystyrene). The No More Nails that was suggested is great and is sticking the coving to my artex ceiling. I have bought a MacAllister coving mitre from B & Q which is suitable for 100 mm and 127 mm coving - it also says that it has a 45 degree and a 90 degree angle. I have now managed to work out how to cut the internal corners, after several mistakes, however, I can't get my head how to cut the external ones? Are external corners cut at the same degree angle as the internal ones, do you have to flip the coving over to cut these corners? I've followed the diagram, so I thought, but the corners don't look right? Any suggestions would be welcome. Thank you.