Window glazing putty belongs to a different era of building—back when individual panes of glass sat in wooden sashes, held in place by strips of linseed oil putty that hardened over time. Modern windows don’t use the stuff. But millions of older windows still need it, and knowing how to work with glazing putty is essential for anyone restoring or maintaining historic houses.
I learned putty work the hard way, on a house with thirty-six original windows that all needed reglazing. The first few were disasters. By the end, I could run a clean bead in my sleep.
What Glazing Putty Actually Is
Traditional glazing putty is ground calcium carbite (whiting) mixed with linseed oil. The oil keeps the mixture pliable during application; exposure to air slowly cures the oil, hardening the putty into a rigid material that bonds to both wood and glass.
Modern formulations add various modifiers—some substitute other oils, add driers to speed curing, or incorporate synthetic polymers for flexibility. The results vary from excellent to terrible depending on the specific product. Traditional linseed oil putty remains the standard against which others are judged.
The confusion between glazing putty (also called glazing compound) and caulk causes problems. Caulk stays flexible permanently; glazing putty hardens. Using caulk where putty belongs creates adhesion problems and looks wrong. They’re different products for different purposes.
When to Reglaze
Glazing fails in predictable ways. The putty hardens and becomes brittle, then cracks and separates from either the glass or the wood. Eventually, pieces fall out entirely, leaving the glass loose in the sash. Water gets behind the remaining putty, accelerating wood decay.
Inspection is simple: push gently on old putty. Sound material feels solid and stays put. Failed putty crumbles, gaps appear at the edges, and sections may pop out completely. Any window showing these symptoms needs attention.
Age alone doesn’t determine failure. I’ve seen original glazing from the 1920s still performing fine, and I’ve seen ten-year-old putty falling apart because it wasn’t applied correctly. Quality of original work and maintenance history matter more than calendar time.
Removing Old Putty
Failed putty must come out completely before reglazing. Leaving old material behind creates problems—new putty won’t bond properly, and trapped moisture causes problems later.
Heat softens old putty dramatically. A heat gun or infrared heater warmed to moderate temperature (too hot risks cracking the glass) makes even rock-hard old putty scrape away easily. Work systematically around the sash, heating a section, scraping, and moving to the next.
Steam works similarly for stubborn areas. A wet rag held against old putty, then heated with the heat gun, creates steam that penetrates and softens the material. The combination of heat and moisture addresses putty that resists heat alone.
Mechanical removal without heat is tedious but sometimes necessary—near thermopane sections or on windows where heat would damage other components. Careful chisel work, patience, and acceptance of slower progress get the job done.
After removing putty, inspect the rabbet—the wooden channel that holds the glass. Sound wood just needs priming. Deteriorated wood may need consolidation or repair before reglazing.
Preparing for New Putty
Bare wood absorbs oil from fresh putty, starving the surface layer and causing premature failure. Prime the rabbet with oil-based primer or linseed oil before applying new putty. The sealer prevents oil absorption while providing a surface that putty bonds to well.
The glass bed—the surface the glass actually sits against—needs a thin layer of putty before setting the glass. This “back putty” cushions the glass and fills imperfections in the rabbet. Spread it thin and consistent.
Setting the glass involves pressing firmly enough to squeeze the back putty to a thin layer without cracking the pane. Small metal triangles called glazing points hold the glass in place while the putty cures. Space points every six inches or so around the pane.
Applying Face Putty
The face putty—what you see from outside the window—requires technique that takes practice to develop. The goal is a clean, angled bead that sheds water while looking neat.
Warm the putty by kneading it in your hands until it’s uniformly pliable. Cold putty won’t bond well and won’t smooth properly. You want the consistency of modeling clay, not cold butter.
Apply putty in sections, pressing firmly into the rabbet corner. Work a comfortable length—maybe six inches to start—pressing and shaping as you go. Trying to run the entire length of a sash side at once usually results in mess.
The putty knife creates the finished surface. Hold it at a consistent angle—roughly 45 degrees—and draw it along the putty bead in one smooth motion. The knife should contact both the glass and the wooden edge of the rabbet, creating that characteristic angled face.
Clean cuts at corners require stopping the knife cleanly without lifting. Practice on scrap until you can end a run without leaving ridges or gaps. The corners show craftsmanship quality more than the straight runs.
Curing and Painting
Fresh putty needs time to cure before painting. Traditional linseed oil putty requires at least two weeks; some formulations specify longer. The surface skins over first, but the interior remains soft. Painting too soon traps solvents and prevents full cure.
Paint is essential—it protects the putty from UV degradation that would otherwise cause rapid failure. Unpainted putty might last five years; painted putty can last fifty. Run the paint slightly onto the glass (1/16 inch) to seal the putty-to-glass joint.
Oil-based primer followed by either oil or latex topcoat works well. Latex directly on raw putty can cause adhesion problems with some formulations. The primer layer eliminates that risk.
Modern Alternatives
Synthetic glazing compounds from companies like DAP and others offer faster cure times and more forgiveness during application. These products work fine for most purposes, though purists and historic preservationists often prefer traditional linseed oil formulations.
Silicone-based products marketed for glazing should be avoided on wooden windows. They don’t paint well and can cause long-term adhesion problems with wood.
The skill of applying putty well translates across product types. Once you can run a clean bead with traditional putty, modern formulations feel easy by comparison. Start with what the old-timers used, then experiment with alternatives if the application warrants it.
