Q I know that blue board is not the same as standard drywall. But if drywall has already been hung, would a Level 5 finish be comparable to plaster skim coat?
A Myron Ferguson, a JLC Live presenter and drywall contractor in Galway, N.Y., responds: Level 5 is the highest-quality finish possible on regular drywall (see “Application and Finishing of Gypsum Panel Products,” gypsum.org), and it is significantly different from veneer plaster on blue board. A Level 5 finish is most often achieved by skim-coating with joint compound, but several companies also make primer-surfacer products that provide a Level 5 finish, such as Tuff-Hide by USG (usg.com).
The most common level of drywall finish is Level 4, in which seams and corners have been taped, then coated (along with fasteners) with two coats of joint compound, and sanded with a fine-grit paper to remove imperfections. To achieve a Level 5 finish, a skim coat of joint compound is applied over the entire surface. This is fairly easy: Joint compound thinned with water is applied to the drywall (I like to use a medium-nap paint roller), then removed with a wide taping knife.
The Level 5 skim coat is intended to conceal slight differences in texture on the drywall surface. Areas that are very smooth from coats with joint compound (seams, for example), uncoated areas where sanding has raised the grain of the paper, and untouched drywall paper surfaces all have slightly different surface textures and vary in porosity. These differences can show even when the surface is painted. Applying a skim coat of joint compound creates a more uniform surface and minimizes the differences in porosity, making the surface look uniform when painted—even in the worst lighting conditions.
A veneer-plaster finish is very different. First, the plaster itself is not the same as joint compound, and when it is applied, it creates a layer of measurable thickness. In addition, plaster actually bonds chemically with the paper in blue board, creating a surface that is much harder than regular drywall with a Level 5 finish. Plaster does not bond well to regular drywall, so using it for a skim coat is not recommended.
If you’ve already hung regular drywall, you can apply one of the primer-surfacer products mentioned earlier. While these alternatives are not the same as a veneer-plaster finish, they have a measurable thickness and provide a harder Level 5 finish than skim coating with joint compound.