Is this dry

Sekekama

A month ago had old plaster bonded/skimmed over. Now this is the result there's some darker patches which feel dry to the touch. I've mist coated two walls and where the patches are it doesn't adhere that well. There's also a crack across the ceiling which the plasterer goes it's because an old house (1935) and old plasterboard beneath. 

FreeBear

The patches might be down to the plaster being polished during the last few finishing passes - Do they look shiny ?

A light sanding with 60 or 80 grit sandpaper would take the shine off. Do it by hand rather than with power tools.

A 1935 house is more likely to have a lath & plaster ceiling originally. Plasterboard only really took off in the 1950s despite being invented around 1917. Skimming a lath & plaster ceiling is always going to be problematic, especially if the old ceiling has cracks in it. Scrim tape helps to hide cracks, but they will often reappear (the same applies to cracked lime plaster on walls). Not much you can do about it beyond filling the cracks and painting over - The other alternative is to take the whole ceiling down (a dirty, messy job) and fixing new plasterboard up. Some say you can just overboard with 9.5mm plasterboard, but is is a crap shoot getting screws in to the joists. Taking the old ceiling down gives you opportunity to add insulation and getting it right up to the walls. Still a very dirty and messy job.

Sekekama

FreeBear said:

The patches might be down to the plaster being polished during the last few finishing passes - Do they look shiny ?

A light sanding with 60 or 80 grit sandpaper would take the shine off. Do it by hand rather than with power tools.

A 1935 house is more likely to have a lath & plaster ceiling originally. Plasterboard only really took off in the 1950s despite being invented around 1917. Skimming a lath & plaster ceiling is always going to be problematic, especially if the old ceiling has cracks in it. Scrim tape helps to hide cracks, but they will often reappear (the same applies to cracked lime plaster on walls). Not much you can do about it beyond filling the cracks and painting over - The other alternative is to take the whole ceiling down (a dirty, messy job) and fixing new plasterboard up. Some say you can just overboard with 9.5mm plasterboard, but is is a crap shoot getting screws in to the joists. Taking the old ceiling down gives you opportunity to add insulation and getting it right up to the walls. Still a very dirty and messy job.

They are not shiny tbh ceiling cracks is hairline so can be filled with polyfilla more worried about patches especially if they show after normal coat of paint atm only mist coated 

Eldi_Dos

Unless you intend to keep it I would lose the carpet and underlay from the room, that will improve ventilation and speed up drying out.

ThisIsWeird

Almost certainly it's either that they haven't fully dried out, or have been polished as FreeBear said. Over-polishing will make these patches hard, smooth, and difficult for the paint to adhere.

Flat sanding block with, say, 120 grit paper, done lightly. You should soon see what the issue is when you do this; if the sanded plaster floats off as a fine powder, then chances are it's dry. And the finish revealed underneath should be an even colour, whether it was previously dark or light.

Since you've gone to the expense of having it skimmed, then I think it would be worth a light going-over in any case, to get the best possible even finish.

Dust mask, goggles, hairnet :-)

Sekekama

Eldi_Dos said:

Unless you intend to keep it I would lose the carpet and underlay from the room, that will improve ventilation and speed up drying out.

It is dry been 1 month 

Sekekama

ThisIsWeird said:

Almost certainly it's either that they haven't fully dried out, or have been polished as FreeBear said. Over-polishing will make these patches hard, smooth, and difficult for the paint to adhere.

Flat sanding block with, say, 120 grit paper, done lightly. You should soon see what the issue is when you do this; if the sanded plaster floats off as a fine powder, then chances are it's dry. And the finish revealed underneath should be an even colour, whether it was previously dark or light.

Since you've gone to the expense of having it skimmed, then I think it would be worth a light going-over in any case, to get the best possible even finish.

Dust mask, goggles, hairnet :-)

I probably should have sanded the wall before mist coat? I've got this https://www.wickes.co.uk/Wickes-Flexible-Sanding-Sponge---Medium-Coarse/p/154894

Eldi_Dos

Sorry for confusion, was going by thread title.

ThisIsWeird

Sekekama said:

ThisIsWeird said:

Almost certainly it's either that they haven't fully dried out, or have been polished as FreeBear said. Over-polishing will make these patches hard, smooth, and difficult for the paint to adhere.

Flat sanding block with, say, 120 grit paper, done lightly. You should soon see what the issue is when you do this; if the sanded plaster floats off as a fine powder, then chances are it's dry. And the finish revealed underneath should be an even colour, whether it was previously dark or light.

Since you've gone to the expense of having it skimmed, then I think it would be worth a light going-over in any case, to get the best possible even finish.

Dust mask, goggles, hairnet :-)

I probably should have sanded the wall before mist coat? I've got this https://www.wickes.co.uk/Wickes-Flexible-Sanding-Sponge---Medium-Coarse/p/154894

All skimmed surfaces need some prep before painting. Sometimes a wipe with a damp cloth is all that's needed, but really a light-going-over with a sanding block will usually pay dividends. For this, I'd suggest the completely flat type I linked to - it'll remove all the wee high-points.

Foam-types can follow existing contours, rather than help level them out, but I'm sure it did its 'cleaning' job fine. If the mist coat looks well adhered on the areas you've done, you should be ok.

DullGreyGuy

Sekekama said:
Now this is the result there's some darker patches which feel dry to the touch. 

Touch is a terrible way to tell is something is dry or not, human skin doesn't have any hydro receptors so we have no inert way of telling if something is wet or not and instead imply its wet based on a combination of temperature, visual cues etc, its why sometimes its really hard to tell if something is damp or cold. 

Sekekama

ThisIsWeird said:

Sekekama said:

ThisIsWeird said:

Almost certainly it's either that they haven't fully dried out, or have been polished as FreeBear said. Over-polishing will make these patches hard, smooth, and difficult for the paint to adhere.

Flat sanding block with, say, 120 grit paper, done lightly. You should soon see what the issue is when you do this; if the sanded plaster floats off as a fine powder, then chances are it's dry. And the finish revealed underneath should be an even colour, whether it was previously dark or light.

Since you've gone to the expense of having it skimmed, then I think it would be worth a light going-over in any case, to get the best possible even finish.

Dust mask, goggles, hairnet :-)

I probably should have sanded the wall before mist coat? I've got this https://www.wickes.co.uk/Wickes-Flexible-Sanding-Sponge---Medium-Coarse/p/154894

All skimmed surfaces need some prep before painting. Sometimes a wipe with a damp cloth is all that's needed, but really a light-going-over with a sanding block will usually pay dividends. For this, I'd suggest the completely flat type I linked to - it'll remove all the wee high-points.

Foam-types can follow existing contours, rather than help level them out, but I'm sure it did its 'cleaning' job fine. If the mist coat looks well adhered on the areas you've done, you should be ok.

Any other sanding option a little block takes a long time