Plasterboard - sous couche or go commando?


I'm about to paint a great deal of new plasterboard - an entire studio, which goes up to 4 m high at the highest point. Opinions are divided about whether I should put a primer undercoat (sous couche) on or not? Those in favour say if I don't, the plasterboard will just suck up the paint and my workload will be doubled anyway. Others have just gone ahead and painted.

Any guidance anyone? Does it make a big difference?

Also, any suggestions for a single coat paint (French preferably) that really DOES only need a single coat?

Thank you for all wisdom and advice...

Yim O'connor's comment, just a couple of comments above.

Not being technical, we tried both and the finish with sous couche way better than without, also used less of the more expensive coloured paint…

I could probably comment on the ventilation issue, Richard, if I knew the precise circumstances. I’ve been away for the past week and have’t seen (and cannnot find) any previous comments on this. Can you give me a url to refer to ?

Perhaps Ian, in view of the previous 'shower room mould' comments, you could also give make some comments about the importance of ventilation?

sous couche

then one coat of mono couch (one coat thick emulsion).


Not entirely sure sealing the interior surface of the plasterboard with waterproof finish is as good an idea as it sounds (though I may have misunderstood!)

I know most will know all this, but I've seen some dodgy practices (particularly by some French artisans) that suggest not everyone does. Some of those on our house and we're having to slowly undo the damage.

Outside walls of old houses were built with water permeable (lime) mortar and possibly with water permeable stone. Over the decades they have quietly sucked up moisture in damp times and dried out in the dry. The lime mortar is softer than stone so when the wall subtly changes size as it heats and cools the changes are accommodated by the mortar.

Inside the house, historically, they tended to have significant through flow of air and water permeable wall finishes so damp escaped from the walls, joined the general humidity created by breathing people, boiling kettles, cooking, drying clothes etc. and escaped with the flow of air through the building.

Now we come along and draught proof the property, trapping any humid air in, and then look for ways of sealing out the moisture.

Pointing and rendering externally with modern cements has risks. The cement can be harder than the stone so as the wall expands and contracts (very slightly) either the new mortar cracks, or the stone itself cracks. potentially damaging the integrity of the wall. This approach also traps any moisture rising from the ground inside the wall. That moisture either has to escape into the house or remain in the wall where freeze thaw action can cause further damage.

Best to let the outside wall breathe (lime mortar and/or render) and insulate and waterproof internally.

The insulation goes agains the wall. Then you put waterproof membrane. Water vapour penetrates from inside as well and will condense if it hits a cold barrier (if the waterproof membrane was between the wall and the insulation the moisture would condense in the insulation.) The plasterboard then goes on the membrane.

If you've done all that there is no need to seal the plasterboard against water penetration on the inside (unless in a specifically wet place like above a bath, sink, etc.) To do so risks trapping any moisture that does penetrate between the waterproof membrane and the outside of the board.

If you haven't done the insulation and waterproof membrane correctly and seal the interior surface instead the plasterboard will simply trap moisture entering from the wall. That will lead to mould inside the board and to it's eventual disintegration.

I have tried just painting emulsion on plasterboard, took 4 coats, and I hate painting. On the next job, using same paint, I put a 50% diluted layer up first, second coat finished the job! Lesson learned.

So, as many others have said, either whitewash, undercoat or (easiest of all) 50% diluted coat of paint to seal the surface.

Absolutely agree: whitewash is the cheapest and most beautiful! But the key is in the 'fixer' : the Sikalatex (or equivalent) is essential. Also be careful when trying to treat walls with emulsion already on them; you'll have trouble getting the whitewash to stick. However, on bare plaster or stone, there is nothing better.

I once whitewashed a whole lighthouse in the West of Ireland with one sack of lime: 2/6!

Suzanne, don’t even think about NOT using a sealer first ! Or you’ll waste a huge amount of time and money. My personal recommendation, from quite a lot of experience in my own home, is a single coat of “Julien sous-couche Universelle” which comes in a 12 litre (silver/black/yellow)can. Not the cheapest to buy, but the difference compared with other products is amazing. You can find it most places but Brico Depot is worth a visit; they often have a 14 litre can for the price of a standard 12 litre. Your finishing coat will then simply glide over the undercoat, and go much farther. As we all know (and Castorama have admitted !) French one-coats usualy need at least two coats. But go for a top quality brand - e.g. Tollens; you’ll pay a bit more but will be in with a good chance of getting adequate cover with one coat, especially if you use the Julien undercoat I recommend. Good luck. Let us all know how you get on. Ian Morris FRICS, Chartered Building Surveyor

Richard, thanks so much for taking the time to spell this out. I spent some time online looking at whitewash a while ago, but there were so many different configurations, I gave up eventually.

If you go straight for your final finish, you'll effectively waste the decent emulsion as it will be sucked up by the very porous plasterboard. I would suggest a coat of cheap primer first, even a second coat two hours later will help with the coverage of your top coat. Then do two or three of that until you're happy with it. The more you get on, the better it will last. If you need supplies or just advice you could contact one of our advertisers here.

Suzanne, not only has plasterboard upstairs, but also some very thin asbestos type covering on internal walls of a small utility and salle douche extension and in the stone built house I rent soaked up damp through external walls, it adds to the moisture content in the house and the mould/spores that have becomes evident on the back of said coverings where now visible are affecting my breathing. Make sure the walls themselves inside have a good waterproof coat of something on them. Moisture and condensation in, although not evident are not good for health - you only want to do it once - so do it properly.

As to what you use and how you do it I have no idea, but Richard G’s reply looks good, if the product he suggests is impermeable!

Thanks everyone - great inputs!

Hi Suzanne

You should always seal new plaster or plasterboard.

The cheapest way to do this and most plasterers will tell you this, is to get a tub of white paint and water it down by 50%, it's what's called a mist coat, once dry, paint the wall/ceiling in whatever colour you choose.

However, it is messy as the paint is very thin, so cover up.

Hi! We had the whole house insulated, and plasterboarded. We were recommended to undercoat first as it seals the boards. We bought the sous couche from the local brico, and it took just two 10 litre buckets to do it all....we only had to paint one top coat over this.........which worked out far cheaper in the long run.

Hello Suzanne, I'm going to propose something which is the opposite of everything you asked for.

Its soooo cheap that one kicks one's self for ever having used anything else!

Buy 1 litre sikalatex (30€) and a 30kg sack of chaux hydrolique (20€) and whichever pigments appeal. You will end up with enough to paint the studio several times over.

Undercoat sikalatex with a very thin mixture of chalk)

Main coat, depends on the style of the result you want. If rustic, make it think, and it'll cover in one coat. For a more modern 'emolsion' finish, a coulple of coats of thinner mix. Add a capsule of sikalatex to th e mix as a binder, of if you need a washable surface, such as for bathrooms and kitchens, probably three capsules per bucket.

You can add the pigment to water, let it rest to avoid 'granules', then add to the mix. Note very carefully the quantities you used, to obtain the same colour with the subsequent batches.

My reasons for loving whitewash are:

- it goes on so easily, and is so easy to clean spills, rollers, spalter etc

- it is so cheap that if you don't like the colour, a thick layer of a new colour and you are away!

- it is cheap and easy enough that you can add layer after layer, and develop a patina, which looks super.

- it does not smell.

"We did 4 25m2 bedrooms with less than one sack, on "fermacell" (its like plasterboard but without all the drawbacks).

I think you will find that, as people have already said to you, plasterboard is 'thirsty' unsess you stabilise it somehow.