Blotchy paint on new plaster

gunner0098

Hi there,

This is my first post on boards so hello to all!

I'm currently building a new house and thankfully we are close to moving in. A strange problem I've come across while painting onto plaster, is that on certain walls the paint is very blotchy. The plaster was left to dry for 3/4 months and has been applied very well. My builder says it's something to do with a chemical reaction with the blocks! Never heard of that before.

I was wondering have any of you come across this and are you aware of a fix?

mad m

If you can, upload a pic. Could be some of the plaster boards were blobbed and it can sometimes look like circles are coming through walls as if they were damp patches....

Does that sound like your problem?

gunner0098

Sorry, should have stated that its plaster on an internal block wall. Will try to get a picture.

chuckles30

When I was painting my house last year I also noticed blotchy patches when I put up the white paint. Once I put up the coloured paint, I never saw the blotches again. Mine were plasterboard walls though, so not sure if that makes a difference.

starman100

Try applying a coat of sealer before painting (unibond diluted in water), we had the same problem I remember, bad batch of plaster meant uneven adsorption of paint.

twenty8

Your builder is right. It is a alkali reaction from the sand and cement - it depends on the level of lime that is in it - different batches will have varying amounts of lime. I presume that it is only on your internal walls - ie the ones that are not insultated and were plastered with sand and cement.

The best solution is to get a alkali primer and put it on before you do any more painting. I suggest that you stop all painting immediately and put this on first. It is hard work and needs to be put on by brush. But once it is on you won't regret it.

gunner0098

Thanks all for your replies.

I got some alkali primer to apply, but the white base coat seems to stop it from working correctly. Obviously needs to go directly on the plaster. (to confirm - it is an internal block wall finished directly with plaster).

I tried appliying the base coat on top of the primer (which is on on top of the origional base coat! - getting confusing now.) but the blotchlyness is still visible. Looks like I'm gonna have to get a good dose of elbow grease and sand the be-jayus out of the walll and start over.
Strangely enough it was really only one wall that is badly affected, with smaller blotches appearing elsewhere.

twenty8

It sounds like it was one batch. Just depends on the batch that came out of the quarry - nothing to do with the plasterer or anything like that. As the base coat is down you should probably sand it down. Tough work - but probably has to be done at this stage.

Some people might say that you can continue painting over it - but it will never really work. I had the same problem and when the light is a certain way it looks like the surface of mars with all the marks on it. At othertimes it looks perfect. Note to self - never hire that painter again!!

Wertz

"Mapping" as it's known is common where bonding has not been used between the blocks and the skim. Most builders seem to just put up a scratchcoat of cement render instead of bonding and where the skim may be thinner in places you get this reaction with the plaster.
The thing about mapping is that you don't see it on the skim until you put paint on it, which is a bit of a catch 22. Polybond will do nothing for this.
Usually we would allow the paint that's been put on to dry fully (at least 24 hrs, preferabbly more) then sand back the poorly adhering parts of the coat on top of the mapping (the reaction seems to leave a shiny finish that emulsion won't take to) and either put undercoat or a stain blocker like BINs on, allpow that to dry and continue painting.

BTW mapping can also be a problem when plasterers use additives to keep the plaster workable longer. The mortar plasticiser can cause the same problem when not thoroughly mixed through the skim...

feorais

Wertz wrote:

"Mapping" as it's known is common where bonding has not been used between the blocks and the skim. Most builders seem to just put up a scratchcoat of cement render instead of bonding and where the skim may be thinner in places you get this reaction with the plaster.
The thing about mapping is that you don't see it on the skim until you put paint on it, which is a bit of a catch 22. Polybond will do nothing for this.
Usually we would allow the paint that's been put on to dry fully (at least 24 hrs, preferabbly more) then sand back the poorly adhering parts of the coat on top of the mapping (the reaction seems to leave a shiny finish that emulsion won't take to) and either put undercoat or a stain blocker like BINs on, allpow that to dry and continue painting.

BTW mapping can also be a problem when plasterers use additives to keep the plaster workable longer. The mortar plasticiser can cause the same problem when not thoroughly mixed through the skim...

If all else fails, ring the technical advice section of Dulux or any of the bigger paint Companies. They have come across all these problems and usually will be able to provide a specialist product that will help you avoid the task of sanding it down. There is even a product that, when applied , allows you to paint over where a tarry substance may escape out through the wall from a chimney! As a matter of fact if you can find this it would probably be ideal for your situation!

Piusno1

I do this painting lark for a living, and I come across this the whole time. Sometimes its very bad and some not so bad, as said only on sand and cement walls. A light Sanding after a tin first coat of emulsion with 120 sandpaper will help on the shiny areas and recoat with a brush but on a whole house you need to be doing it for urself, as it takes time. Putting a tin coat of acrylic emulsion on first with a 6in brush will help a little, never use a roller as rolling over the same area more than once makes it a whole lot worse. Brushing you have more control. Once the first coat has cured, should be left for at least a week or more , then you can roll on two coats tinned emulsion of your desired colour be carefull not over roll, It will not be perfect the light might show it up( daylight)
The only other way is spray on a mist coat and give it time to cure and repeat.
Hope this helps. Joe

gunner0098

Thanks for all your replies folks.

Originally Posted by Piusno1
never use a roller as rolling over the same area more than once makes it a whole lot worse.

Well this is exactly what I did but under advise from the person in the paint shop!! When this just proves what they know!

I ended up putting two more coats of base and two coats of top on it. It came out ok but if you view the wall at an angle you can still see the "mapping".

I did start another wall, but I gave it a rigorous sanding first, and it came out perfect after just one coat of base. This is the approach I'll be taking to the remaining rooms in the house. Hopefully it won't be to bad anywhere else. At least the dormer rooms are plaster board!!!