Random and very dry patches appear when plastering some of my old walls

[Harrybrice]

I'm totally confused by what I witnessed last night when skimming one of my walls. I am laying onto old sand and cement render, I made sure to do two coats of PVA to reduce the suction as much as possible.

When I started applying my first coat I suddenly had what looked like random patches of extremely high suction where the plaster dried out instantly, I'm 100% sure that I hadn't missed these spots when PVA'ing. I hought that maybe I hadn't laid the plaster on thick enough so kept adding more thinking that it would stay wet at some point, it didn't really help.

I ended up with blotches of instantly dried plaster right next to newly completely wet spots, this made closing the wall in and getting it flat a challenge. I ended up doing the second coat and managed to get a half-decent finish but the instantly dry spots ended up being humps in the wall, unfortunately.

It's not the end of the world, but I just don't understand how this could have happened - Have you ever had anything like this before?

 

[Stevieo]

My old grandad used to say, 'Eeh, pva. Tha can never have too much pva.....but tha can have too little'

Actually he didn't because he was an engineer. But I bet he would have if he was a plasterer.

 

[Harrybrice]

To be fair, the PVA had fully dried by that point, maybe I should just stick to laying on when it's only half-dried (tacky).

Thanks for your hypothetical Grandad's advice...

 

[Retired Spread]

Gyprime woulda sorted it.....to expensive for most people though

 

[Harrybrice]

Gyprime woulda sorted it.....to expensive for most people though

Yes, looks like it's top stuff but these patches of S&C render are pretty small and wouldn't justify a large tub of the stuff. I found a suggestion to use bitumen paint when skimming over a wood frame (had no choice) that was joined to plasterboard, and it worked really well, don't see why it wouldn't do the same for very high suction patches...

 

[ChrispyUK]

Yes, looks like it's top stuff but these patches of S&C render are pretty small and wouldn't justify a large tub of the stuff. I found a suggestion to use bitumen paint when skimming over a wood frame (had no choice) that was joined to plasterboard, and it worked really well, don't see why it wouldn't do the same for very high suction patches...

Thin the pva down and keep applying till it sits on top.



Then put your bitumen on









Not