Preparing the Walls

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New York Times

HOME CLINIC; The Technique of Skim-Coating a Plaster Wall

By EDWARD R. LIPINSKI
Published: March 14, 1999

PLASTER walls frequently develop a network of hairline cracks that are too narrow to be filled with patching plaster, yet they cannot be covered with paint. The best way to restore the wall surface is by skim-coating it. Skim-coating is the technique of applying thin layers of gypsum-based joint compound over the entire surface. The layers are so thin that even after applying several coats, the total thickness of all the layers together is less than an eighth of an inch. Skim-coating can also be used to smooth a textured surface.

Skim-coating does not require any arcane skills. It is a matter of troweling on the coating, then spreading it thin with a wide, drywall taping knife. The job, however, is time-consuming; it will take three four days to skim-coat the walls of an average-sized room. It is also rather tedious.

Skim-coating can rescue a wall with numerous hairline cracks but cannot help a wall if the plaster is loose or unsound. Test the condition of the wall first by pushing against it with your fingertips. If the surface gives under slight pressure, it may mean that the plaster has broken loose from the lath keys behind. In this case it is only a matter of time before the plaster cracks and drops off in chunks. The remedy here is not a light surface coating, but replacing the loose sections with drywall.

If the walls are firm with perhaps an occasional crack or hole, then you can skim-coat them. You will need to collect your tools and materials first. Skim-coating is done with drywall compound: a white, pastelike substance that is applied to drywall to cover joints, cracks, and nailheads. For the first coating, use setting-type compound. It is available in 18- or 20-pound bags in powdered form and must be mixed with water. Consult the package specifications for information on coverage and also to see if the compound can be sanded after it dries — some compounds set rock-hard and cannot be sanded.

The package will also give you an indication of working time — how long it takes for the compound to dry. Usually this is indicated by a number that follows the brand name. Thus a brand like Stay-Smooth 45 will set in about 45 minutes. Unless you are a fast and skilled worker, it is best to choose a compound with a 90-minute setting time.

It is important to use setting-type compound for the first layer or base coat, because it dries by chemical action that will not affect any paint that is on the wall. Air-drying mixtures, like ready-mix joint compound, can weaken a paint bond causing the paint and skim-coating to fall away eventually. Once the base coat is on the wall, however, you can use ready-mix joint compound for the second and third layers.

You will only need a few simple tools to skim-coat. A 6-inch-wide and 10-inch-wide taping knife will be sufficient, unless you need to work in a few narrow areas, then you may need a knife with a 3-inch blade. A useful, but not necessary, tool is a hawk (see illustration). You use it to hold the compound as you work around the room. You will also need a large five-gallon plastic bucket to mix the setting-type compound.

Start by making sure the wall surfaces are clean and free of grease and dust. Remove any crumbling plaster. You can use the setting-type compound to fill large cracks and holes but first paint any bare, unpainted areas of plaster with shellac. The shellac will seal the plaster and keep it from absorbing water from the compound.

Mix the setting-type compound according to the package directions. Use a broomstick or a garden trowel to stir the water and powder together until it attains a consistency of soft ice cream. If the mixture is too thin, it will run down the wall. Compound that is too thick will be difficult to spread.

Start in the corner of the room at the top and work down toward the center. Then move to the bottom and work up. Use long, smooth strokes and overlap each pass. Using this technique, gradually work your way across the wall. Pause occasionally to wash off the knife. The base layer should be relatively free of bumps and ridges, but it doesn’t have to be perfectly smooth.

Allow the base coat to dry — this may take several hours — then apply the second coat. Use the 10-inch knife to smooth the compound into a thin, even layer. If the second layer is smooth, two layers may be sufficient; if not, add a third after the second layer is dry. The final step is to sand the entire surface, then prime and paint it.

FINE HOMEBUILDING

Ready mix Joint Compound comes in a variety of formulas including all purpose, lightweaight and with dust control.In this Building Skill we’re going to talk about ready mix mud and I’ll show you how to mix it so it goes on easily and spreads smooth.
Here are some qualities to consider when choosing between types of drywall mud:

All-purpose joint compound

  • Sands to a smooth finish
  • Resists dents and scuffs
  • Is the heaviest type of compond

Lightweight joint compound

  • Is about 20-lbs. less per 5-gallon pail than regular compund
  • Sands with less effort
  • Doesn’t sand as smooth
  • Scratches easily

Dust control

  • Sheds heavier dust when sanding, which tends to fall to the floor instead of staying airborne
Skim Coating FAQs
 
Do I need to prime prior to skim coating?
In many cases, you will want to start off by priming.
 
You should prime if:
  • walls are glossy (prime with oil primer—Coverstain) See Bonding Primers.
  • drywall is torn or damaged (use a specialty primer like Zinsser’s Gardz for this)
  • the paint is peeling (address the peeling, then prime with oil primer—Coverstain)
 
However, if the wall is painted in flat paint, you shouldn’t need to prime.
 
What type of compound should I use?
In most situations, you will be skimming the walls twice. I use a chemical setting drywall compound for the first coat. This sets quickly so that I can do both skims in one day.  The second coat needs to be easy-sanding, and it can dry overnight. I use an all purpose or a topping compound for this. (USG Plus 3 works well.)
Skim Coating FAQs
 
Do I need to prime prior to skim coating?
In many cases, you will want to start off by priming.
 
You should prime if:
  • walls are glossy (prime with oil primer—Coverstain) See Bonding Primers.
  • drywall is torn or damaged (use a specialty primer like Zinsser’s Gardz for this)
  • the paint is peeling (address the peeling, then prime with oil primer—Coverstain)
 
However, if the wall is painted in flat paint, you shouldn’t need to prime.
 
What type of compound should I use?
In most situations, you will be skimming the walls twice. I use a chemical setting drywall compound for the first coat. This sets quickly so that I can do both skims in one day.  The second coat needs to be easy-sanding, and it can dry overnight. I use an all purpose or a topping compound for this. (USG Plus 3 works well.)
 
 

Our “terms of use” governs your use of our website; by using our website, you accept this disclaimer in full.  If you disagree with any part of our “terms of use”, do not use our website.
Many painters / paperhangers have had a problem or two over the years when painting / paperhanging over “hot” (chemically reactive- quick setting) drywall compounds.  I am not a chemist but I will attempt to explain what I think is the underlying reason(s) for the difficultly, and the rules that I use with quick setting drywall compounds.
The Internet Paint Store
 
“the right way to buy
paint supplies”
 
 
 http://www.paintinganddecoratingconcourse.com/articles/14.html
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Mixing quick set mud
Use a clean five gallon bucket and mix using an electric mixer and drill. Mix the compound well. Add more water if the mix is too dry. Add more compound if the mix is too loose. The mixture should be loose enough to spread easily, but not so loose that it won’t stay on the knife. Scoop the mixed compound out of the five-gallon bucket and put some in the mud pan.
 
Cover four by four-foot areas at a time. Starting with the top half of an eight-foot wall (for instance), apply a thin “skim” coat of compound from the ceiling down to the middle of the wall.  Overlap the next strip and do the same until you have done a four-foot wide swath.  Next, do the lower half of the wall, applying a thin skim coat from the bottom up to the middle of the wall, joining the lower section to the upper section that you just completed.  Repeat this until you have completed a four-foot wide swath on the lower half.  Repeat the above procedure until the entire wall (or area) is skimmed out.
 
The second coat
Allow the first coat to set for the required amount of time (or longer). You may then apply the second coat, using regular all-purpose or topping compound.
 
All-purpose compound comes pre-mixed and dries slowly, usually taking overnight. You don’t need to worry about this compound hardening in the pan as you do with chemical setting compound.
 
There are two ways to do the second coat:
1. Do exactly what you did on the first coat—simply repeat with the easier sanding all purpose compound.
2. Cross-apply the compound. That is, apply the compound in a horizontal direction (to the length of the wall), as opposed to the vertical application that was done on the first coat. If the wall is wavy, this is the best way to do it.  However, most of the time repeating the second coat vertically is fine and turns out quite well.
 
Allow the second coat to dry overnight (minimum), then sand. As you become skilled at skim coating, you will find that the sanding required is very light, as your walls will be pretty smooth before sanding.
 
I use a sanding attachment with my shop vacuum and a HEPA filter (and fine dust collection bag) for virtually dust-free sanding of the skim coat.
 
Vacuum the wall and remove all sanding dust, then prime with a PVA primer, and you are ready to paint with any paint of your liking.
 

How to Skim Coat a Wall After Wallpaper Removal

By Julie Hampton, eHow Contributor

Read more: How to Skim Coat a Wall After Wallpaper Removal | eHow.com http://www.ehow.com/how_5637368_skim-wall-after-wallpaper-removal.html#ixzz2FeM081QH

Removing wallpaper can be a tedious and lengthy process. After all the paper and glue have been stripped from the wall, a severely dinged and damaged piece of drywall may be left. Repair chips and dents by skim coating the wall with lightweight joint compound. The process takes several steps, but the results are a smooth, new-looking wall.

Read more: How to Skim Coat a Wall After Wallpaper Removal | eHow.com http://www.ehow.com/how_5637368_skim-wall-after-wallpaper-removal.html#ixzz2FeMIMvID

Things You’ll Need

  • Medium-grit sandpaper

  • Rag

  • Putty knife

  • Joint compound

  • Drywall trowel

Read more: How to Skim Coat a Wall After Wallpaper Removal | eHow.com http://www.ehow.com/how_5637368_skim-wall-after-wallpaper-removal.html#ixzz2FeMQZcpB

Instructions

    • 1

      Sand the entire wall using a medium-grit sandpaper. Wipe the wall with a damp rag to remove any drywall dust.

    • 2

      Use a putty knife to apply a small amount of joint compound onto the edge of a drywall trowel. Holding the trowel at a 90-degree angle, apply a light, even coat of plaster onto the surface. Do not apply more than 1/4 inch of plaster at a time. Fill in all holes and dings on the wall’s surface using the trowel and joint compound. Compare the technique to icing a cake or buttering toast. Allow the first coat of plaster to dry about four hours.

    • 3

      Lightly sand the dry plaster with a fine- to medium-grit sandpaper to remove any loose plaster. Wipe off any dust with a damp rag. Add more wet plaster to the wall with the trowel, allow it to dry and continue the process until all grooves have been filled in and evened out. Several layers of joint compound may be necessary.

Read more: How to Skim Coat a Wall After Wallpaper Removal | eHow.com http://www.ehow.com/how_5637368_skim-wall-after-wallpaper-removal.html#ixzz2FeMXE1U0

There are a couple of common ways to mix compound:

  • A heavy-duty drill fitted with a paddle or auger style mixing attachment is the easiest and most efficient method
  • A large “potato masher” mixer works if you don’t have a heavy-duty drill, but it requires more physical effort

Comparing Joint Compounds

Most Joint Compounds are made from the same material but the consistency between the compounds varies.

Top Coats        Base Coats

It just depends on what you want to do with the compound.

Skim coat
A thin coat of joint compound spread over the entire surface to fill imperfections.

All-purpose compound
Used for setting tape and covering joints and fasteners, it is a combination of drywall joint and a topping compound. Premixed or packaged as a dry powder to be mixed with water, it has many of the smooth-spreading qualities of topping compound but has greater adhesion.

Topping Joint Compound
Topping is a smooth sanding material for second and third coats over all-purpose compound. Produces excellent feathering and superior finishing results.

Tape with all purpose. It has more adhesive in it and will hold the tape better. Coat with lightweight. It is easier to sand.

Two Types of Joint Compounds

Setting Compounds    and

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