In the pursuit of paint perfection, every surface flaw must be tackled before the first fog of basecoat ever leaves the spray gun. Among the tools in your arsenal, glazing putty takes center stage for its ability to provide precision and detail. While other fillers excel at bulk filling and structural support, glazing putty’s finesse lies in erasing minor scratches, pinholes, and shallow dents.
However, glazing putty (sometimes called spot putty) is often used incorrectly by novices as a one-size-fits-all solution. And the outcomes are predictably undesirable, usually revealed as low spots or cracks. Understanding the strengths and limitations of this specialized filler can prevent common errors for optimal results on your own projects.
What is glazing putty?
Glazing putty and body filler both serve similar purposes, but they differ considerably in their purpose, composition, and application. Body fillers are intended to level out large and/or deep imperfections, as well as help shape contours. Putties, conversely, are meant to fill light scratches, small pin holes, and low spots that emerge during sanding and priming.
Though both products are typically polyester-based, glazing putty contains a higher proportion of solvents to achieve a thinner consistency. And while polyester body fillers almost always use a catalyst or hardener, some spot putties harden up without one. Putties often cure faster as a result of their higher solvent content, which can also lead to greater shrinkage.
While body fillers are typically laid on thick – it’s not called “mud work” for no reason – glazing putty needs to be applied in multiple thin layers to achieve any kind of thickness. Putty’s lighter composition, however, allows for easier sanding and allows it to feather with other primers and fillers more easily.
Common Putty Mistakes
Glazing putty should be used sparingly and for a limited number of uses. These include surface smoothing after body filler but before primer, feathering edges between repaired and original surfaces, small touch-ups revealed during sanding, or final post-bodywork repairs. Below are the ways putty is typically misused.
Deep Filling When used for deep filling, it can shrink, crack, or fail to provide adequate support.
Primary Filler It lacks the structural strength of regular body filler and may lead to poor adhesion, cracking, or sagging.
Moisture Exposure Applying glazing putty to surfaces with moisture or humidity can affect the curing process, leading to poor adhesion and potential long-term issues.
Skipping Priming Applying glazing putty directly to bare metal without proper priming can lead to poor adhesion and potential rust issues. Glazing putty is not designed to protect against corrosion or bond with bare metal.
Overuse Thick layers may take longer to dry, increase the risk of cracking, and make sanding more challenging.
Not Sanding Between Layers Applying subsequent layers without proper sanding between coats may result in an unsmooth finish where adhesion between layers can be compromised.