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stratocaster1422 07-01-2009 06:32 AM

Prepping new plaster walls
The wife and I just recently had blueboard/plaster guys in to do the walls and ceilings of our living room. While they did an amazing job, there are a few very small areas that are either slightly rough compared to the rest of the walls or some very light trowel marks. I have a few prep/paint questions below:

1.) Should I do anything to smooth these areas out prior to prime/paint? If so, what grit sand paper? I was thinking the wet sand grit, but doing it very lightly and dry. I was also thinking that the primer/paint thickness would pretty much hide any slight imperfections.

2.) Any specific primer better than others for on top of fresh plaster? I typically use Killz when working with sheetrock/joint compound. I was thinking I didn't need this type of primer and could just use generic primer.

3.) The ceiling plaster is textured with a light drag. What is the best/easiest way to go about painting it? My thought was a very heavy knapp on a roller.

Nestor_Kelebay 07-01-2009 02:53 PM


When you say "plaster", do you mean a true lime putty based plaster like the kind used on houses built with plaster walls, or do you mean a gypsum based drywall joint compound?

The gypsum based stuff can be primed with any primer.

Lime putty based plasters, like fresh concrete, are far too alkaline for oil based primers, and a PVA latex primer won't do well on them either. For true lime based plasters, you need to either leave the plaster unpainted for at least a year while the lime in it reacts with CO2 in the air to form limestone, or prime it with a special acrylic primer made specifically for painting fresh concrete.

Nestor_Kelebay 07-01-2009 03:26 PM

regarding the rough areas, what you should do is hold a bright light (60 watt bulb or equivalent in a darkened room is good enough) close to the wall beside those rough areas so that you can see whether they would be better smoothed by ADDING joint compound or by sanding those areas down.

I've heard of people using 220 grit sandpaper to sand joint compound, but I think that's dumb. I typically use 100 grit sanding screens, and the primer and paint coats completely fill in any roughness of the substrate to dry to a smooth coat, so I don't see any point in using anything finer than a 100 grit sanding screen.

the primer and paint MIGHT cover any roughness in these areas, but you need to keep in mind that both primer and paint go on fairly thick, but dry to a relatively thin film as the water or thinner evaporates as the paint dries. So, it's a toss up. If it wuz me, I'd fix those areas before priming and painting when they're easiest to fix.

Nestor_Kelebay 07-01-2009 04:56 PM

Re: Using KILZ as a primer.

You CAN use KILZ as a primer, but it's really not an appropriate use of that product. An ordinary general purpose latex primer or interior alkyd primer would be a better choice.

KILZ is nothing more than a conventional interior alkyd primer that uses a rapidly evaporating thinner. The idea here is that if the primer becomes too thick for a stain to diffuse through quickly enough, then you can encapsulate the stain within the primer film. Conventional alkyd primers will use 100% mineral spirits as the thinner. KILZ uses a mixture of 60 percent naptha and 40 percent mineral spirits as it's thinner. Naptha is camping fuel, and in order to keep a good cooking flame going, the fuel has to be volatile enough to evaporate rapidly. In fact, naptha evaporates about 5 times as fast as mineral spirits. KILZ uses that rapid evaporation of the naptha to encapsulate any stains bleeding through the wet KILZ film so that the stain doesn't diffuse all the way through the wet KILZ film to discolour it's surface (and hence, "bleed through" the KILZ film).

Becoming thick enough fast enough that a stain won't diffuse all the way through the KILZ film is an effective way at stopping stains from bleeding through KILZ, but it's also an effective way to make a primer that you can't apply with a brush without making a brush stroke mess and tearing your hair out. Essentially, KILZ thinner evaporates so quickly that you have to be very conscience of getting the stuff down quickly (within a half minute) and then leaving it alone or you end up with the product so thick that you can't spread it without making a mess of it. Really, about the only successful way of applying KILZ is with a paint roller.

In your case, I would suggest using any general purpose latex primer or interior alkyd primer instead of KILZ.

You might be thinking: Well, I'll use the KILZ cuz I'm gonna apply it with a roller anyhow. If you do that, then I'd encourage you to buy a quart of any general purpose primer to do the interior corners (where you can't get in with a roller) and for any cutting in you'll need to do.

I don't see a problem using one primer for the corners and cutting in, and then doing everything else with KILZ using a roller as long as you allow time for the first primer to dry completely before applying the KILZ. If it was me, I'd do all the priming with a conventional latex or interior alkyd primer.

Nestor_Kelebay 07-01-2009 05:07 PM

Painting a lightly textured ceiling can be done with a paint roller. If it's lightly textured, I'd try using a 3/8 inch nap roller first, and only go to a thicker nap if the roller isn't getting all the way in to the texture.

I use 3/8 inch nap rollers on smooth walls and ceilings only because they hold more paint than the 1/4 inch nap rollers typically recommended for smooth surfaces. That's cuz the thicker nap holds more paint and that means I can paint a larger area each time I reload the roller with paint.

stratocaster1422 07-01-2009 06:13 PM

It is blue board with Uni-Kal veneer plaster. It will have 2wks of cure time by the time I get to paint.

The safest approach to not wait a year would be an acrylic primer, then whatever latex paint I want? Are Acrylic an Alkyd primers the same? I'm a little confused.

I also saw on a California Paints spec that you should sand after applying the primer. I think the plasterer mentioned something like this also.

Nestor_Kelebay 07-01-2009 10:22 PM


Originally Posted by stratocaster1422 (Post 31931)
It is blue board with Uni-Kal veneer plaster. It will have 2wks of cure time by the time I get to paint.

I would talk to your plastering contractor to see if this is a lime based plaster. If it is, then two weeks is not nearly enough time for the alkalinity of the plaster to subside, and the only way to paint it would be to use an acrylic primer meant for fresh concrete. See what your contractor recommends as he'd know more about the problems (if any) associated with painting it than I would since I have no experience with the stuff.


The safest approach to not wait a year would be an acrylic primer, then whatever latex paint I want?
Yes. But, don't just use ANY acrylic primer, use a special acrylic primer made especially for painting fresh concrete. It will use acrylic resins that are the MOST resistant to highly alkaline substrates. But, see what your contractor says. If he's less than forthcoming about how long it will take to be confident that you can prime and paint it, I'd use a fresh concrete primer just to be on the safe side.


Are Acrylic an Alkyd primers the same? I'm a little confused.
No, there are two basic kinds of paint chemistry, oil based and water based. In this post I will be refering to water based paints as those that are typically considered "latex" paints. There are water based epoxy and water based polyurethane paints that wouldn't be considered "latex" paints.

Oil based primer and paint chemistries can be broken down into:

1) drying oil coatings - like linseed oil, Tung oil and the pre-polymerized oils like Danish oil and Swedish oil, and

2) alkyd coatings - which is what 99 percent of "oil based" primers and paints are nowadays. An alkyd resin is best though of as a "clump" of drying oil molecules (or parts thereof) which, by the miracle of modern chemistry have been made to crosslink much more densely so form a harder film much more quickly than the old linseed oil based paints ever could.

2 1/2) Polyurethanes - are a particular kind of alkyd resin that dries to an even harder film at the same rate as a regular alkyd resin would.

Drying oil coatings like linseed or Tung oil, alkyd primers and paints and polyurethanes all have poor resistance to alkalinity because the plant oils they're made from (like linseed oil, soy bean oil and Tung oil) are similar to the plant oils that soaps are made of (like palm oil and olive oil). And, one of the oldest chemical reactions known to modern man is the making of soap by combining animal fats or plant oils with an alkaline material to make soap through a process called "saponification". This is in fact how the Romans made soap, but there is some question as to whether or not they used it for cleaning (as we do) or just as a topical skin ointment or medication (like the sap from the aloe vera plant). If you paint over fresh concrete with an oil based coating like linseed oil, alkyd primer or polyurethane varnish, then that coating will deteriorate rapidly as the natural plant oil components in the coating are converted into a crude form of soap through that same "saponification" chemical reaction. It takes at least a year for the alkalinity in fresh concrete and fresh lime based plaster to subside enough to be able to paint them without concern about the alkalinity wrecking the paint, and two years wait is better.

Latex primers and paints, on the other hand, can be broken down into two primary types:

A) Those where the "binder" or the stuff that forms the solid film of the primer or paint is made from tiny clear solid particles of the plastic "polyvinyl acetate". This is the same kind of plastic used to make white wood glue, and so primers and paints made from a polyvinyl acetate have some characteristics similar to dried white wood glue in that they remain slightly sticky even when they're fully dried and they deteriorate under moist or wet conditions. In the industry, primers and paints made from polyvinyl acetate are called "vinyl acrylic" or "PVA" primers and paints.

B) Those where the "binder" or the stuff that forms the solid film of the primer or paint is made from tiny clear solid particles of the plastic "polymethyl methacrylate". This is the same kind of plastic used to make Plexiglas, and so primers and paints made from polymethyl methacrylate have some characteristics similar to Plexiglas, notably that they form a harder plastic film that's highly resistant to UV light from the Sun, (which is why greenhouse windows are made of clear Plexiglas), has good resistance to acids and alkalis, and is not at all sticky once it's fully dry. Basically, Plexiglas type primers and paints are better at everything than white wood glue type primers and paints except at being less expensive. In the industry, primers and paints made from Plexiglas plastic are called "100% Acrylic" or "Acrylic" (for short). These kinds of primers and paints also stick better to damp or moist surfaces.

B 1/2) There are also paints made with something called a styrenated acrylic binder, but they're more common in Europe than they are in North America.

So, since oil based primers of all kinds have poor alkali resistance, you don't want to use an oil based primer on fresh concrete or fresh lime based plaster. Water based primers and paints have better alkali resistance, but of the three different kinds of water based primers and paints, the Acrylic ones made from the Plexiglas type plastic (polymethyl methacrylate) have the highest alkali resistance. And, if you buy a primer made to be used over fresh concrete, you will be getting a primer that is made from the polymethyl methacrylate resins which have the very highest resistance to an alkaline substrate. That is, you want to buy a "100% Acrylic" primer that's made specifically to be used over fresh concrete (or fresh lime based plaster).

Nestor_Kelebay 07-01-2009 11:16 PM

You don't need to read this post. It just explains how oil based and latex paints form films. At this point you don't need to know about that, but I decided to post in case anyone else reading this thread was interested.

Film formation in Oil based primers and paints:
ALL oil based and alkyd paints form a film by cross linking when they are exposed to oxygen in the air. Oxygen molecules (O2) react at something called "unsaturated sites" on the drying oil molecules. When an O2 molecule encounters two unsaturated sites in close proximity either inside the same drying oil molecule or alkyd resin or between two drying oil molecules or alkyd resin, it breaks apart to form two separate oxygen atoms which each form a C-O-C crosslink at that site. Thus, what WAS a pair of unsaturated sites in close proximity now becomes a pair of C-O-C crosslinks at that location. It is the formation of these pairs of C-O-C crosslinks within the oil based film that gradually converts it from a viscous liquid into a solid over the course of anything from a few hours to a few days.
When oil based primers and paints are exposed to their air, the weight of the primer or paint actually increases while it's exposed to air because of the absorbtion of O2 molecules from the air into the primer or paint. It's the O2 molecules that are absorbed into the primer or paint while it's still in the can that causes oil based primers and paints to form a "skin" while in storage. Water based paints will never form such a skin.
You can buy cans of compressed nitrogen at Lee Valley. The idea is that you fill the empty space inside the can of oil based paint with nitrogen prior to closing it up to prevent the formation of that skin on the paint. The problem is that this will only work well if the liquid paint isn't exposed to the air for long because it's absorbing O2 all the while the can is open. Blowing in some nitrogen before re-sealing the can isn't going to stop the O2 that's already been absorbed into the paint from forming a skin while that can is in storage. Instead of paying for this compressed nitrogen, you can use a butane canister made for recharging butane lighters. Butane is heavier than air and should settle to the bottom of the air space inside the can. But ultimately, to prevent a skin from forming over oil based paints in storage is to minimize the exposure of the paint in the can to the oxygen in the air by any means available to you.

Film formation in Latex primers and paints:
All conventional "latex" primers and paints form a film through a completely different process called "coalescence".
Basically, a latex paint is a SLURRY consisting of tiny solid coloured particles (called "pigments") and tiny clear solid plastic particles (which are the "binder resins") suspended in a solution of water and a water soluble solvent called a "coalescing solvent".
When a latex primer or paint is applied to a surface, the first thing that happens is the water evaporates from the wet film. As the water evaporates, the plastic binder resins find themselves surrounded by the
coalescing solvent at an ever increasing concentration. As the coalescing solvent concentration increases, the coalescing solvent becomes absorbed into the clear plastic binder resins and causes them to swell up and become soft and mushy and sticky. The same forces of surface tension and capillary pressure that cause small water droplets to form large rain drops in a clowd then take over to cause each soft mushy sticky binder resin to stick to and pull on it's neighbors. As that happens, the paint film transforms from gazillions of swollen mushy sticky clear plastic "blobs" spread on the substrate like a pile dead jellyfish into a film of swollen mushy sticky "polyhedra" (which is a three dimensional solid with many sides that don't have to be parallel to each other). This is the form bubbles will take inside foam; polyhedra with flat sides, the flat sides being their interface with neighboring bubbles.
After this coalescence of the binder resins into a solid continuous plastic film takes place, then the coalescing solvent gradually evaporates from the paint film causing the soft binder resins to return to their original hardness and creating that "freshly painted smell" in the room. That "freshly painted smell" is just the smell of the coalescing solvent.
The final product is a continuous hard film of clear plastic with the coloured pigments suspended inside it very much like raisins in raisin bread.

If you look carefully you'll notice that latex paints darken significantly as they dry. The reason for this is because prior to coalescence, the wet paint film contained gazillions of tiny blobs of clear plastic (the binder resins) suspended in water. Incident light was both reflected and refracted at the surface of each binder resin because the refractive index of the plastic the binder resin is made of is different than that of the mostly water solution surrounding it. Your eye sees the light scattered from the surfaces of all these binder resins as "white" light, just like you see snow banks and clowds as being white even though nothing inside either one is white in colour. As the latex paint forms a film through that coalescence process, the liquid/solid interfaces disappear and so the amount of white light you see coming from the paint film dimishes and the paint appears to darken. The colour of the pigments in the paint hasn't changed, it's just that you're seeing less "white" light caused by the scattering of incident light as the plastic paint binder forms a film, and it's this reduction in the amount of white light your eye sees that makes latex paints appear to darken as it dries.

There, now you know more about both oil based and latex paints than most people working in paint stores.

Nestor_Kelebay 07-02-2009 12:05 AM

I kinda doubt you'd be able to find a latex or oil based primer that's "sandable".

Wood workers will often use a "sanding sealer" as the first coat of clear finish they apply to wood furniture. The idea is that the first coat of any finish will cause any loose fibers on the wood to stand up or curl up, and this can make the surface of the first coat of finish rough. By using a sanding sealer as the first coat, the woodworker can sand that first coat down to remove any lumps and bumps caused by the wood fibers rising and curling. Thereafter, the woodworker will use polyurethane or varnish for each subsequent coat of finish.

I am not aware of any "sandable" oil based or latex primer. So, I'd smooth the rough sections before priming them when they're easiest to smooth by either applying more plaster to them or sanding that bare plaster down. If you end up using a primer made to be used over fresh concrete, you almost certainly won't find one that's "sandable". The non-sandable ones you do find will just gum up your sandpaper or sanding screen so they can't be sanded (from a practical standpoint).

So, I'd play it safe by getting the wall smooth first, and then prime and paint.

GBR 07-04-2009 10:02 PM

Various job conditions such as suction differences, wet or only partially dry walls, and
reactions between paint and lime may cause unsatisfactory paint finishes, particularly
on new construction. Alkali-resistant primers specifically formulated for use over new plaster will permit decorating with oil or latex type paints. Quality paint products should be used and manufacturers’ recommendations followed. Finished plaster should be painted or covered to conceal possible discoloration. The paint system should be
suitable for use over plaster surfaces that contain lime, which has a high pH of 10-
It is essential that plaster be sound and completely dry before painting. Under good drying conditions, veneer plaster may be painted 48 hours after application.
High build, heavy duty and special purpose coatings such as Epoxy are not
recommended over veneer or job gauged lime putty finishes.
In all cases, the paint manufacturer should be consulted and approve paint system suitability for use with gypsum/lime finish plaster.
This from Uni-Kal, page 12: Be safe, G

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