How to plaster a patch and obtain a nice smooth seamless joint.
Plastering patches is something that can look simple due to it being a small area. But the reality is that they are one of the trickiest things to do - not only do you have to control the suction and carry out the full 6 stages, , but you also have to be skilful when joining plaster .
Don't underestimate the complexity of patching - allow yourself plenty of time and hopefully all will be well.
There can be a tendency to want to finish the patch immediately but depending upon the suction it may need time to firm up before you can carry out the next stage. Just skimming the patch can take up to 2 hours (on a low suction backing). But if the backing coat is high suction you may need to go through all the stages really quickly and after 10 minutes you could be finished - so the suction will play a massive part.
If when you run your fingers over the firmed up patch - you can feel a ridge, then you will probably see it when painted. Keep troweling to smooth it out and feather in with a very small amount of plaster to fill any hollows.
On smaller walls it can be easier to apply the base coat flush with the edges, then apply PVA to the whole wall and re-skim the entire wall.