Sanding down the current plaster. Focusing on the edges so they taper off slightly towards the exposed brickwork (assume this will help the new and old plaster to form a better single surface)
Not necessary to taper, just break away the edge of the plaster with a hammer to make a ragged edge; straight edges not a good idea. If its vinyl or gloss paint on there, I use an old scraper to score the paint through in a criss-cross pattern & then a very stiff wire brush to provide a good key. Gloss paint needs either bonding primer or brush on a mix of PVA & Bonding plaster or you could use cement or even sand & let it dry thoroughly before plastering over.
Pva coat the bare brick.
First bonding coat.
Second bonding coat.
Bonding is really for low suction backgrounds so you must put a couple of sealing coats of PVA over the brickwork & let it dry off overnight, especially if it highly absorbent block work. Apply another 2 coats of PVA in quick succession & the 1st Bonding coat as it goes tacky. Get the 1st coat as flat as you can but don’t worry too much as it’s likely to go off pretty quick even with the PVA; you can keep it working with a little water from a kitchen spray bottle but don’t use too much or it will affect the plaster. You can then concentrate on getting a much flatter 2nd coat as it won’t go off as quickly as the first.
Smooth with the rest of the wall then allow to dry.
You should ideally skim the Bonding just after it’s gone off or at least the same day or it will suck the water from the skim coat & you’ll be in all sorts of trouble. If you can’t, put a sealing coat of PVA over the bonding & let it dry off overnight, as above, & then another 2 coats of PVA skimming just as the second starts going tacky.
You don’t need to sand down the joins just blend in the Bonding with the existing plaster & stick some reinforcing tape over the joins before you skim; the finish skim coats will take care of the rest.
Pva coat entire wall.
Skim coat x2.
PVA 2 coats as above; it’s normal to use the same mix for both 1st & 2nd coats but as you’re not experienced you will almost certainly struggle for time; using DIYnewbee99’s tip of using a fresh mix for the 2nd coat will help. Don’t use too much water in an attempt to keep the plaster going or you’ll ruin the surface; once it’s gone, better to leave it & use some filler on the problem areas afterwards.
(insert praying after each step)
You won’t have time for that
If i mess up it will be a learning experience, so hopefully second time around will improve...and i doubt it could look any worse than it currently is.
You’ve obviously taken the trouble to find out about it before attempting & that’s a big + but don’t expect too much from your 1st attempt. The experience will be very useful & enlightening &, you never know, you may turn out to be a natural which could be of considerable benefit to your current predicament; good luck.
