I’ve been thinking a lot about chemistry lately. Specifically, the chemistry of coatings (like paint) and substrates (like plaster). There’s a lot of chemistry involved, the kind of thing we don’t think about when we paint a room or repair damaged wallboard. There are the restrictions and limitations we’re familiar with: wait 2 hours (or more, or less, depending on the paint) between coats; don’t apply latex paint directly over wet compound. But there are other things most people, other than professional painters and plasterers, don’t know about applying paint, patching plaster, and more. How successful a particular wall repair will be comes down not just to the skill of the workman, but to the chemistry of the products he uses.

This is all front-of-mind for me because we recently moved to a 130-year-old house, with beautiful wood detail and solid plaster walls. Over time, the walls will show cracks that might widen as the house settles and plaster detaches from the wood lath underneath. Sometimes exterior walls have no lath, where plaster was applied directly to the brick. Many of the cracks are new, having appeared when our contractor used a sawzall to restore pocket doors that didn’t operate well, or at all. Opening up a few windows in the wall — to install a modern trolley-track — was a necessary step, but not one I wanted to watch. My mood wasn’t helped when I asked them to please exercise a bit more care and they flatly denied that the cracks had anything to do with the shaking walls.

Other cracks and deteriorating plaster are due to other things: leaks through the front wall or a flood from above, for example. In many cases the crack or blister — where a quarter inch of compound and paint visibly separate from the wall — is an older patch that was poorly applied. Chemistry: absent an application of a bonding agent to the older, drier plaster, the new patch will eventually separate when moisture is wicked out. Two pieces of sandwich bread aren’t going to stick together without a layer of peanut butter in between.

The contractor is gone now, and has been for months. And I’ve been repairing the cracks as I go, fixing each area before painting. I’ve learned a lot, and mostly about the chemistry of plaster: when to apply a bonding agent, which patching compound to use in each situation, where to use caulk instead of compound, and how to reattach plaster to the lath. This is a critical step in repairing cracked walls, because unless the plaster on both sides of the crack is stabilized — rendered immobile — the crack will return. No amount of gauze tape and drywall compound will ever hide it forever. But I’m confident, now, that the cracks near the pocket doors won’t reappear.