Painting Plasterboard

[andycatax]

I have just taken on a unit and want to just paint the new plasterboard i dont want to spend money on getting it plastered. Is it possible to paint it directly? would i need to use a primer first?

 

[Zampa]

Thin the first coat of emulsion out by about 30%...then paint as normal...rubbing down the wall with 80/100 grade abrasive paper between each coat to remove the 'furing' effect casued by the fibres on the surface of the plasterboard swelling up due to the water based paint.use matt emulsion

Or if you mean its an industrial unit and yur not to fussed about the walls dont worry about the rubbing down

Use matt emulsion not silk or satin...they will show every deviation on the wall.

 

[andycatax]

Hi Zampa, Yes its for an industrial unit and thank you for the advice.

 

[Nestor_Kelebay]

Andy:

Here in North America it's standard practice to paint plasterboard. (Most of us call it "drywall", but occasionally you hear it referred to as "plasterboard".) We simply fill and tape the joints, and plaster over the screw or nail holes, and then just prime and paint over that.

Since we don't skim coat the whole wall with plaster before priming and painting, I'm not sure that our "drywall" is exactly the same as your "plasterboard". Our "drywall" has a smooth finished white paper on the exposed side and a coarse brown paper that goes against the wall studs.

Anyhow, we don't thin our emulsion primers. Back in the days when paints and primers were made with linseed oil, then it made sense to thin those products with turpentine when painting wood because the slower drying time meant deeper penetration of the linseed oil molecules into the wood, and therefore better adhesion of the primer to the wood. However,
we don't see any benefit in thinning an emulsion primer at all, except it saving you a bit of money.

If you look on a can of emulsion primer, and it says that it's both a primer AND a sealer, then those words are important.

PRIMING means sticking to a surface and providing an easy-to-stick-to surface for a subsequent coating to stick to.

SEALING means preventing the migration of substances into or out of the substrate you apply the sealer over.

An emulsion primer that also says that it's a sealer means that it will have huge massive rocks in it that are almost large enough to see with the naked eye.

The purpose of these rocks is to plug up the relatively large holes in joint finishing plaster (compared to the size of holes in paper) so that the primer doesn't all get wicked in to the finishing plaster. You want to plug up those holes in the finishing plaster so that a thick enough film remains over both the finishing plaster and paper so that none of the subsequent coat of paint gets wicked in to either one. That way, you're assured of a uniform appearance of the paint over the whole wall despite the fact that the plaster is more porous and will absorb more liquid than the paper.

Without those huge rocks (which are called "extender pigments" and serve other purposes in paints and primers as well) then the finish plaster over the joints between the plasterboard sheets would wick in all the primer. So, when that primer dried, you'd see bands of dull areas where all the primer was wicked into the porous plaster, and less dull areas where the primer wasn't wicked completely into the paper.

So, look on your can of emulsion primer and see if it says it's also a sealer. If so, it will contain those huge rocks and is suitable for use over both taped plaster joints and drywall.

But, as I say, I don't see any reason for thinning the emulsion primer, other than to save some money. (But, our emulsion primers might be different than yours, just as our "drywall" may be different than your "plasterboard".)

Also, we don't sand (or as you say, "rub down") the primer. By it's very nature, primers will contain enough extender pigments (albeit smaller rocks than those used for sealing) that the surface of the primer will be rough enough so that any paint will stick well to it.

If your "plasterboard" is similar to our "drywall" the smooth surface paper won't lift or swell as a result of using an emulsion primer over it.

The reason why furniture finishers will sand between coats is not to improve adhesion, but simply to remove any blobs or glitches that would cause a blob or glitch in any subsequent coat. Frequently, furniture finishers will only sand after every second coat to save time and work. Also, it is common to use a "sanding sealer" over new wood when first finishing. Every first coat of finish will result in some raising of loose wood fibers, and sanding sealers are made so that they can be sanded down without gumming up the sandpaper. Typically, a sanding sealer is used over the bare wood, allowed to dry and then sanded smooth, and then subsequent coats of finish are applied over that, with every 2nd or 3rd coat sanded down to remove anything that interferes with achieving a smooth finish coat.

 

[Zampa]

If your "plasterboard" is similar to our "drywall" the smooth surface paper won't lift or swell as a result of using an emulsion primer over it.

It is..and it will....maybe we use a different finish paper here..but there is still a tendenct for it to 'fur' slightly

We dont have 'emusion primers' over here as such Ness...only water based primers commonly known as Acrylic pirmer undercoat...have you got a link for any of the brands of emulsion primer that are common over there...im interested to se what they are about.

And do you use a stuff called drywall sealer...or plasterboard sealer on plasterboard?..its a water based 'paint' that is intended for use under emulsion on plasterboard...its meant to protect the surface of the plasterboard if its stripped after being papered.

As for sanding sealer...ah great stuff, but sadly hardly ever used except by french polishers...or maybe painter if they are finishing with cellulose laquer.

 

[Nestor_Kelebay]

...have you got a link for any of the brands of emulsion primer that are common over there...im interested to se what they are about.

And do you use a stuff called drywall sealer...or plasterboard sealer on plasterboard?..its a water based 'paint' that is intended for use under emulsion on plasterboard...its meant to protect the surface of the plasterboard if its stripped after being papered.

The primer I use for bare drywall and plaster is Zinsser's Bullseye 123:

http://www.zinsser.com/product_detail.asp?ProductID=11

It's got the big rocks in it to prevent excessive wicking into bare plaster.

Typically, we just finish the joints with tape and mud over the joints and nail or screw heads, sand smooth, put a coat of primer/sealer on, then paint over that with the top coat.

What we use before wallpapering is a product called "sizing", which I'm not familiar with. The sizing is painted on the wall and allowed to dry prior to papering the wall. By doing that, it makes removing the paper much easier in future, and saves any damage to the bare drywall or paint on the drywall.