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If your home was built before the 1940s and was not remodeled in the past 50 or so years, you probably have plaster walls. Aside from assuming, there are some practical ways to tell if you have plaster walls, including the following: If you knock on the wall and it feels solid, not hollow, if you can’t insert a thumbtack easily, if there are cracks that look like spiderwebs, or if your home has curved, arched, or swirled walls, doorways, or window openings.

If you have an unfinished basement or attic, you could examine the backs of any finished walls to check, too.

You can skim coat plaster, but it’s better to repair the plaster itself. A skim coat is a thin coat of drywall joint compound spread over the surface of a wall. The result is a flat finish that looks pleasing, but part of the appeal of plaster is its imperfections and aesthetic. Skim coating is best when limited to drywall applications.

All plaster walls will crack over time due to normal house settling and expansion and contraction from temperature and moisture changes. Small, thin hairline cracks are generally not serious and are mostly a cosmetic issue. However, larger or more significant cracks can indicate structural problems.

Here's how to tell the difference:

• Cosmetic Cracks: Cracks that are 0.5 cm (0.2 inches) or thinner are typically cosmetic.

• Structural Cracks: Cracks wider than 0.5 cm, excessive or deep cracks, multiple cracks in the same area, or cracks that span around corners can signify a structural or foundation issue. Cracks along the joints where walls meet the ceiling also warrant attention.

For cracks wider than 0.5 cm, structural repair is needed. If a crack is 2.5 cm (1 inch) or wider, or if you notice any signs of a structural problem, it is best to contact a plaster repair professional to assess the damage.

Drying alone can take 1 to 4 hours to dry to the touch per plaster layer, but some types of plaster take several hours longer to dry. However, this doesn’t mean the job is done because plaster doesn’t fully harden until it cures, which can take a few days to a couple of weeks. It may take even longer if you’re building a new, pretty thick plaster wall. Allow the plaster to cure before painting it.