Patch repair damaged plasterboard

Couple of areas in my house I need to repair - one shown below:



Had to hack off some of the multi finish on this one, what would be the best way to repair? The paper layer has also come off the plasterboard so I can't reskim the current surface.

Would it be blue grit and then multi finish?

Do I need to use wickes Bonding Agent?
https://www.wickes.co.uk/Wickes-Ready-Mixed-Plaster-Bonding-Agent---3kg/p/221807

Or a bonding coat instead of blue grit?
https://www.wickes.co.uk/British-Gypsum-Thistle-Bonding-Coat-Plaster---25kg/p/220055

Could I use easi filler instead of multi finish?

 

jamie765, good evening.

Was the damaged area wet / damp?

Ken.

 

Hi Ken,

The area was not wet. However there was paint bubbling in this area, as well as a few other corners on ground floor walls so I paid for a damp survey last week (by a very knowledgeable surveyor who had no damp proofing to sell).

He peeled off paint where it had bubbled and revealed lots of patches of white filler which the decorator had generously slapped on the plaster - he said the issue is due to condensation and this filler being used on the walls when they were first painted 3 years ago. As there is only underfloor heating on the ground floor (which never goes right up to walls) and no wall radiators, some walls have cold spots and this is where the problems are. In the areas where paint has bubbled, I've since scraped off loose paint and found white patches of filler behind them all - if I run my finger over them they are powdery.

He advised just to scrape off loose paint, sand down the problem areas to get rid of all filler, mist coat and repaint. Whilst the walls aren't wet to the touch he did say drying them with a fan heater wouldn't do any harm (he advised a dehumidifier wouldn't be as useful in this scenario).

However this particular wall, along with one other, are the worst - the plaster had blown / loosened off these walls, so I had to hack it off. Just not sure what the best way to deal with it would be