We’ve all seen it, and we’ve all done it. To mount an HVAC duct, we’ve cut through a plaster wall with a recip saw, sending plaster flying and leaving the lath flapping. No problem, the drywallers will handle it, right? Wrong. The most durable plaster repairs are made with plaster, not joint compound. Plus, without the proper prep work on loosened plaster, any patch is doomed to failure.