Mist coat coming off with masking tape

[dianef]

We've painted new plaster with mist coat (Dulux Trade Supermatt) over a month ago. The mist coat was mixed as per instructions (3 parts paint 1 part water) and applied with a roller/brush. The colour was surprisingly rich for a mist coat but it went on ok.

A month later, when preparing the walls for the next coat, I've noticed that masking tape will fetch the paint off when removed. The masking tape is 3M painter's tape (blue stuff medium tack). Paint doesn't come off when scratched but masking tape just strips it straight off leaving clean plaster behind.

The walls have been thoroughly vacuumed and brushed and plastering was done months ago - it was completely dry. Any ideas what the problem is and what to do next?

 

[Richard C]

Who did the plastering? If it was over polished or you’ve got PVA contamination (but you can usually see that) you won’t get good paint adhesion.

I never use anything fancy for mist coating, just simple cheapo “own brand” matt white emulsion. Thin 25-30%, apply 2-3 coats in quick succession before they have a chance to dry out, leave 24 hours & apply your chosen paint finish.

 

[dianef]

So what do I do now?
One thing I was surprised at when applying mist coat - it was very rich and full - one coat and the wall was hardly patchy - it looked finished solid white

 

[Richard C]

As I said I don’t use anything special for mist/priming coats & it still looks very wishy washy next day to the point the plaster colour shows through underneath!

Sand it off & try again; sanding (but don't go mad) should also take care of any over polishing if that was the problem.

 

[dianef]

Hmm, not keen on sanding and re-paining if I can help it. See, thing is I am not sure what I did wrong. I used good paint (trade supermatt), I watered it down to exact quantity (3-1), cleaned the walls etc. Plaster is very smooth but not to the point where paint wasn't adhering to it.

Visually it all looked ok, except I imagined mist coat to be quite washed out and its not - its full on white. Where the paint has come with masking tape, underneath plaster is clean - like paint was a film on top rather than soaked in.

I could spend all week sanding it down and repainting and come up no better though. I am also not sure I want to change paint - I got another 3 tubs of supermatt to use and most people would agree I shouldn't have any problems with it.

So if I am sanding it down and re-painting - should I water supermatt down more or something else? I have to say I find supermatt very fat and creamy even watered down it was very thick.

 

[misterhelpful]

I have to say I find supermatt very fat and creamy even watered down it was very thick.

This could be the problem and it may have needed to be watered down a bit more. Dulux paints are notoriously thick and only experience helps with knowing how much each type needs to be watered down. (This may sound a bit cynical, but, even though the tub gives a guide as to dilution ratio, Dulux want to sell as much paint as possible so don't really want to advise you water it down too much. )
The bare plaster may well have absorbed too much of the moisture from the thick SuperMatt leaving it sitting on top of the plaster rather than penetrating it, as you suggest.

I'm with RichardC in saying that a cheaper brand matt emulsion, preferably Contract Matt, should be used (although, SuperMatt is basically an expensive, thick contract matt and is suitable for misting and finishing new plaster, hence the solid colour you mention), but if you want to continue using the SuperMatt after sanding back, you can only try watering it down a bit more so that it resembles single cream.

One ray of hope is that many people don't mist coat at all and are lucky enough not to encounter problems unless they stick things to their walls. If you only applied tape to do 'the tape test' and don't plan on sticking anything to the walls in the future (including wallpaper), then you might be one of those lucky ones and you may get away without sanding back. (Don't tell anyone I said that, mind! )

 

[dianef]

Thanks.

See, thing is, I didn't just follow Dulux instructions, I followed advice on here and I haven't seen anyone suggest I water more than 3-1. I'd put in 3 liters of Dulux and 1 liter of water and paint is still so rich that I can cover the entire wall with one coat with almost nothing of plaster shows through.

I am not sure what to do next - sand back and apply half paint and half water? I've not heard of anyone having to water down paint that much though which is why I am a bit confused.

 

[Richard C]

I am not sure what to do next - sand back and apply half paint and half water? I've not heard of anyone having to water down paint that much though which is why I am a bit confused.

If you search the forum you will find references to folks thinning this stuff down 50% & even 60%; personally I can’t see the point, you might as well use cheap matt for the initial mist coats.

What has happened is the paint pigment has sat on the surface of the plaster rather than the plaster sucking it into the minute pores on the surface as it absorbs the water; this is what gives it a key & good adhesion. Over polishing by the plasterer won’t help either & some relish a “glass like” finish which is if fact the last thing you want. You may get away with it as stated but if adhesion is poor, the paint can start blistering & flaking off on it's own accord once it’s fully dried out (could take a year or so) let alone coming off in sheets if you stick wallpaper over it.

 

[dianef]

I spoke to Dulux - their advice was stop using the masking tape if its fetching the paint off. Not very helpful, eventually they agreed to take a sample and analyse it.

Problem is - what Dulux are saying is that you don't want to be watering the paint down too much. Their view is if you mix too much water in the paint, the water will be sucked into the plaster and the paint will sit on the surface. Kind of whats happened here. Either way, it looks like we are **** - more than half the house is already painted with the stuff and I don't know whether to ignore it and carry on or start sanding it all back room by room and then do what?

 

[misterhelpful]

You may get away with it as stated but if adhesion is poor, the paint can start blistering & flaking off on it's own accord once it’s fully dried out (could take a year or so) let alone coming off in sheets if you stick wallpaper over it.

Exactly right, which is why I added my 'disclaimer'.

@dianef

It's understandable your confusion because, as said, the dilution ratios in the forum differ greatly, but as RichardC says, in some cases people will use a 50/50 mix. I can only assume this is with paints like SuperMatt (I haven't used it for a while so it's consistency may have changed in the meantime but I know it's price seems to have rocketed!) as standard contract/matt emulsions would be far too thin (for my liking anyway) to use when diluted this much and the general rule of thumb is around 25%.

I'm afraid the ultimate decision in what to do next will be up to you but sanding back and re-misting would be the correct way and the most likely to give lasting results.

Good luck with it, whichever way you go.

Edit:
As Richard says below, paint that is too thick will just sit on the surface and peel in no time.

 

[Richard C]

Their view is if you mix too much water in the paint, the water will be sucked into the plaster and the paint will sit on the surface. Kind of what’s happened here.

I don’t know who you are talking to at Dulux but that’s wrong, did you not read my post! It’s the water being sucked into the plaster that pulls the paint pigment into the plaster pores with it; if the paint is too thick or the plaster over polished, it will just sit on the surface which is exactly what you have.

 

[Growler]

I've never put masking tape over plaster that was mist coated over a month ago.
I doubt whether any other painter and decorator or plasterer has either.

Anybody

 

[misterhelpful]

I've never put masking tape over plaster that was mist coated over a month ago.
I doubt whether any other painter and decorator or plasterer has either.

Anybody

Can't say that I have and I try to avoid putting tape on any paint at all but in theory it shouldn't make a difference as the paint should have been absorbed into the plaster.

 

[Growler]

In theory.

 

[misterhelpful]

In theory.

True.