preventing paint reactions

[garmcqui]

Afternoon all,

I’ve stripped wallpaper from walls in two bedrooms, ready for painting. The walls were originally painted before wallpaper. The paper came off ok, and i sugar soaped and made sure all the adhesive was off. Filled and sanded down anywhere where paint had come off etc.

But when painting with ordinary matt emulsion, I keep getting bubbles/blisters, like the attached:



I can only think it’s a reaction with the original paint (~25yrs old). Once it’s dry, i am scraping the blisters off and the paint comes off back to the bare plaster. So i’m filling, sanding and repainting. It’s a nightmare.

I have other rooms to do next, and want to avoid similar problems. I’ve read that an oil-based primer/sealer would be my best bet? Then emulsion on top. Can anyone confirm this? Which one would be best?

Thank you in advance.

 

[sparkwright]

It could be that in the beginning the new plaster wasn't mist coated first.

It could be that the paste, the sugar soap, the water, and the new emulsion has softened the original paint so it just pulls off.

 

[garmcqui]

Thanks.
So what is the best thing I can put on to seal the old paint from the new, to try prevent reactions? I’m presuming an oil based sealer or primer, maybe something like BIN?

Don’t know if I can be bothered with lining paper.

 

[opps]

Initially, I assumed that it might be residual glue causing the problem but that should not result in the original paint blistering. It may be the case that the first coat of the original paint was not thinned sufficiently. If that is the case then oil based undercoat or BIN will help. Both will create a "waterproof" coating that will prevent the water in the new emulsion from penetrating through the old paint.

The downside to oil based undercoat is that the room will stink for 2 or 3 days and you may need to wait a week or so before you can apply the emulsion over it. The length of time depends on the emulsion. If you apply it too soon you may get "fisheyes" in the emulsion.

Being alcohol based, BIN will cure much faster. You will be able to overpaint it on the same day. It too will stink but the smell will go an hour or so once it becomes touch dry. It is however the more expansive option. BIN can be tinted and makes a very durable finish for walls if you want to apply two coats of it.

Another option which may work is applying a dilute coat of SBR (it is a waterbased waterproofing product. 5L will cost £20. I normally dilute it 3 parts water to 1 SBR. That would give you 20L of dilute SBR. The mix will be pretty much as thin as water. If it doesn't cause the paint to blister, then you may be OK. Belt and braces, apply a second coat before the emulsion. The mix might well be enough to seal 4 or more rooms. 5L of BIN will cost about £80 and cover about two rooms.

Whichever you go for, be advised that the first coat of emulsion will take longer to dry because the paint can only dry outwards (ie in to the room). I don't anticipate that you will have any problems but in areas of high humidity, eg, poorly ventilated bathrooms, the emulsion may craze.

Personally, I would SBR the walls and line them, but I appreciate that you don't want to do that.