Re-keying an old lime scratch/float coat for skimming

[mdfx]

I started trying to remove some wallpaper and a gypsum skim coat started coming off. In some small areas the whole lime base coat has fallen off. Ideally I'd like to patch in new lime base coats where I have to, and re-skim the rest of the float coat with a lime finish. But I'm worried the key on the wall won't be sufficient to hold the top coats.

Is there a way people commonly "re key" old plaster? Do I just set into it with a drywall knife?

 

[Lacan07]

I started trying to remove some wallpaper and a gypsum skim coat started coming off. In some small areas the whole lime base coat has fallen off. Ideally I'd like to patch in new lime base coats where I have to, and re-skim the rest of the float coat with a lime finish. But I'm worried the key on the wall won't be sufficient to hold the top coats.

Is there a way people commonly "re key" old plaster? Do I just set into it with a drywall knife?

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Well that lime surface looks pretty keyed up as it is! You don't have to go mad to get lime to stick. Just make sure the wall is dampened down properly before you skim.

To produce a key you can make your own Devil Float. Buy a wooden float (Wickes do them) and drill some screws through the back so the tips are protruding out the front. You can use this to rough up the surface. I would use the hardest screws you can find as the lime will soon blunt them and you'll need to keep changing them out for new.

Looking at that photo I wouldn't say the 'whole' of the base coat has come off. Just dub out any major holes with some base coat so they're level and then skim.