Paint Mapping - help!

[RobJWill87]

Hi folks,

We had damp proofing work done in the living room a month ago, and we've given the new plaster plenty of time to dry out nicely. This weekend we've begun painting, and we started with a mist coat of watered-down matt white emulsion. Looking around online the advice for the consistency of the mist layer ranged from 50/50, to even 90/10 and so we aimed at a happy middle, 75% paint to 25% water. We immediately noticed areas where the paint was sitting in what you could describe as raised areas, creating a horrible embossed effect. After the mist coat dried we did a second emulsion layer, hoping that this would then even out, but the problem is still there. Looking around online we found out that this is commonly referred to as paint 'mapping.' There are various reasons for it apparently, but it seems the defect is in the plaster, with usual reasons being that it's been either overworked or dirty water has been used in the mix.

I just sanded down an area in one of the alcoves and basically it seems that where the paint doesn't sink in properly it comes off fairly easily with light abrasion, though the surrounding paint seems to sit properly -

Result of a light sand

mapping.jpg (87.44 KiB) Viewed 7127 times

Really hoping there might be some wise folk out there who can suggest an easy fix for us please, I'd hate to have to take it all off, and even if we did that, I'm not sure what we would do to avoid the issue on a subsequent painting attempt? I've tried to contact the company to ask for advice given that it's their plasterwork, but as its a Sunday I don't really anticipate a response. We've got laminate floor being installed in the week and had hoped to get all this done before then to avoid accidentally wrecking the new floor.

Many thanks in advance

Rob

[OchAye]

There are various reasons for it apparently, but it seems the defect is in the plaster, with usual reasons being that it's been either overworked or dirty water has been used in the mix.

I just sanded down an area in one of the alcoves and basically it seems that where the paint doesn't sink in properly it comes off fairly easily with light abrasion, though the surrounding paint seems to sit properly -

Good morning Rob. I thought to try and respond yesterday but I would rather someone who has had similar experience replied. I have not had the problem (I am a diyer). Anyhow, what you got is patches of highly polished plaster. The usual recommendation here is to gently abrade the plaster. You need a hand sander example and 180?? 150?? or 120?? abrasive paper (the lower the number the rougher the paper). You don't want to cause sanding lines on the plaster but you need to take the "sheen" of it.

Also, the more polished the plaster the more diluted the mist coat must be. What paint have you been using?

EDIT: Try not to mistcoat areas where the paint is holding well (esp. if your second full coat was with vinyl paint), and let the mistcoat dry in full before the next full coat. It can be other things than highly polished plaster but I am trying ot keep it simple. If you are really pushed, zinsser gardz will crack it for you but the plaster must have dried out.

[RobJWill87]

Many thanks for the reply, really appreciate it. I rang the damp proofing company to complain today, and the boss came around to see it this evening. He took one look at it, apologized, and offered straight away to have the boys back to do another skim. As it stands they'll be back next week to do it, and I've got everything crossed that in a few weeks when it's dry again we'll have better luck with it. He even offered to reimburse us for the paint we've wasted. So far so good, because I couldn't have asked for a better response to it really.

I'll bear your advice in mind in case somehow this is the same next time!

[Have Brush Will Travel]

Good advice from Ochaye

Re your mix for mist coat..thats pretty much bang on..90:10...assuming they meant paint to water is ok if you use a retail matt..they are thinner anyway

Id never go 50:50...thats half and half (some people get it muddled with 50% water...it isnt)

[RobJWill87]

The plot thickens because I realised that the mapping effect has only occurred on the lower walls where the wall was rendered after being hacked off for the damp course work. It’s also occurs much higher up on one wall to the level of the top of door frame, which is where they hacked it off higher to do the treatment work. That wall is by far the worst offender, and looks like this -

mapping.jpg (255.54 KiB) Viewed 6896 times

Further to the boss having popped around last week, the plasterer came this morning, and as I was expecting, he was very defensive about it. Don’t get me wrong, I can understand why; he’s obviously very proud of his work.. Initially he tried to say it was an issue with the paint job because he said he’s never seen or heard of ‘paint mapping’ before, but he did relent and start listening properly when he took my point that the mapping effect has literally only occurred to the height of where the wall was hacked off and rendered. You can even see a line where it stops occurring, and then the skim on the rest of the walls above those areas is perfect. He wouldn’t be drawn into speculating on the cause of it though…

Is it perhaps simply then a case of over-polishing on those parts, or does the fact that it’s only on the rendered bits point to something else? I’m curious as to those other potential causes Ochaye if you have a min to let me know please.

After a bit of heated discussion, they’re coming back to reskim tomorrow, and fingers crossed that’s an end to it.

[OchAye]

Good photo. So what are we looking at?

Mist coat.
Top coat and then you have the mist-coat that failed.
Then you sanded down some parts where the mist coat failed.

Then what have you done?

PS. Good on you they are replastering.

[RobJWill87]

Good photo. So what are we looking at?

Mist coat.
Top coat and then you have the mist-coat that failed.
Then you sanded down some parts where the mist coat failed.

Then what have you done?

PS. Good on you they are replastering.

That’s about the sum of it. That latest pic is just to show the state of play after a mist coast, undercoat of white and the first layer of light grey on that wall. The mist coat immediately showed this effect in the areas affected as described, but not knowing what it was we had tried to paint it out with the subsequent layers. We used Leyland matt white emulsion from B&Q. I’d left the alcove wall alone for now where I’d experimented sanding etc (the pic in my first post). Hadn’t done anything since your last response ahead of waiting to see what the result of them coming back to see it would be, but thankfully they’re okay to just re-skim. The fact that it literally only shows on the rendered areas says to me there must be a specific reason why it’s happened there in particular. Ultimately, it should now be fine with a new skim tomorrow, but given the hassle of it and the heated discussion this morning I’m very curious as to the why it happened in the first place. Is overworking still the likely culprit do you think?

[Bob225]

It looks like they have used pva or an additive in the skim - assuming they tanked the walls it could be the additive in the render and the plaster has sucked it up, The paint will never stick without it a primed/blocker being used

[OchAye]

That’s about the sum of it. That latest pic is just to show the state of play after a mist coast, undercoat of white and the first layer of light grey on that wall. The mist coat immediately showed this effect in the areas affected as described, but not knowing what it was we had tried to paint it out with the subsequent layers.

Yes, that is what was asking ... the subsequent layers. The subsequent layers would not cover the step between "the sea and the land/islands on the map". But did the subsequent layers stick were the mist-coat had failed?

We used Leyland matt white emulsion from B&Q. I’d left the alcove wall alone for now where I’d experimented sanding etc (the pic in my first post). Hadn’t done anything since your last response ahead of waiting to see what the result of them coming back to see it would be, but thankfully they’re okay to just re-skim. The fact that it literally only shows on the rendered areas says to me there must be a specific reason why it’s happened there in particular. Ultimately, it should now be fine with a new skim tomorrow, but given the hassle of it and the heated discussion this morning I’m very curious as to the why it happened in the first place. Is overworking still the likely culprit do you think?

Bob maybe onto something or the rendered areas were still wet but wet would not cause the problem that quickly. It does not look like highly polished plaster, that would come in large(r) patches "I guess". Whilst I am on guessing mode, Leyland "Trade" would have a maximum dilution of 10%, Leyland retail who knows. So unless Bob has got something there, I think it is the mist coat that failed. But if Bob is correct then there is a likelihood the next mist-coat would fail too.

[RobJWill87]

Don't see that it can be down to the mist coat to be honest, reason being that the rest of the wall where the effect isnt showing is perfect, and we did the entire room in one go. If it was that, the problem would be everywhere I'd have thought? Its too much of a coincidence that its only where it was rendered, so I'm thinking Bob must be right?