Glazing compound is a product which is used in the installation of glass panes. The process of fitting glass panes into a window, known as glazing, includes applying glazing putty to create a snug seal which will keep water out. People often need to replace the compound when a window is broken or when old compound starts to crack and peel. For installing new windows, many people use fully glazed and prehung windows available through building suppliers, in which case they just need to fit the frame and window into place, and do not need to worry about glazing.
Historically, glazing compound was a putty product, and traditional putties are still available. It's also possible to find various rubber and caulk products designed for use in window glazing. In all cases, the compound must be somewhat unique, because it needs to be able to make a tight seal between two different surfaces, the glass of the window and the wood, plastic, vinyl, or metal of the frame.
When glazing compound is first applied, it can be slightly sticky, gradually skinning over in a day to a week, depending on the product, so that it can be painted. However, the compound will remain flexible even after it has skinned over, which is actually a beneficial trait, because it gives as wooden frames contract and expand, ensuring that the seal around the window is not broken. This keeps water out and reduces the risk of mold and eventual rot around the window frame.
If glazing compound starts to crack and flake off, it can be removed with a putty scraper or similar tool so that a fresh layer of compound can be applied. Stubborn compound can be heated with a tool like a hair dryer to soften it if it will not give easily. When replacing broken windows, it's important to remove the glass, glazing points, and glazing compound, sanding the frame down to smooth the wood so that it can be sealed with a fresh layer of primer before replacing the window.
Once the frame has been prepared, a thin layer of compound can be applied before pressing the glass gently into place, taking care to confirm that the compound is evenly spread. Then, the glazing points, the small metal clips used to hold the glass into place, can be attached to the frame, and another layer of compound can be applied to seal the glass and glazing points into place. Some people do this by hand, wearing a glove to protect their fingers from sharp edges and keep the mess to a minimum, while others use a thin putty knife to apply it. Then, the glazing compound should be allowed to cure before painting the window frame.