When applying repair putty in thick sections, a major problem arises when the surface layer cures fully—forming a “skin”—but the deeper material remains soft, uncured, or weak. This phenomenon is a direct result of the exothermic curing reaction and inadequate heat dissipation, leading to a structurally compromised repair.

1. Controlling the Exothermic Reaction (Heat Management)

The chemical reaction of two-part putties generates heat (exotherm). If this heat is trapped in a thick mass, it accelerates the surface cure while hindering the deeper cure.

• Staged Layering (Thin Lifts): The most critical solution is to build the repair in thin layers, or “lifts,” generally no more than 1/2 inch (12mm) to 3/4 inch (19mm) thick, depending on the putty. Applying in thin layers allows the heat from the curing reaction to dissipate into the air and the metal substrate before the next layer is applied.
  • Procedure: Allow the previous layer to reach a “green” or partially cured state (solid but still workable) before applying the next. This prevents cumulative overheating.
• Small Batch Mixing: Mix only the amount of putty needed for the current thin layer. Mixing a large volume generates more initial heat, shortening the working time and causing the material to thicken quickly—a process known as “bulk heating” or “mass effect”—which accelerates the surface skinning.
• Spread Out the Mix: After mixing a large amount (if necessary), immediately flatten the mixed putty onto a cool, non-porous surface (like an aluminum or steel plate). This dramatically increases the surface area, allowing the initial heat to dissipate and extending the working time before the skin forms.

2. Optimizing the Curing Environment

The environment around the repair influences how quickly heat can escape the putty.

• Use a Cool Environment: Whenever possible, perform the repair in a cooler ambient temperature (within the manufacturer’s specified range). Lowering the surrounding temperature slows the chemical reaction rate, reduces the peak exotherm, and allows for more uniform temperature distribution throughout the thickness.
• Heat Sink Contact: If the part is small, place it on a large, cool metal table or slab (a “heat sink”) during the cure. The metal will actively pull heat away from the putty, reducing the temperature differential between the surface and the core.
• Air Circulation: Ensure there is good airflow around the repaired area. Moving air helps whisk away the heat radiating from the surface, preventing localized overheating and surface-only curing.

3. Application Technique for Uniformity

How the putty is placed into the void can affect the cure.

• Avoid Trapped Air: When packing the putty into a deep void, use firm, repetitive tamping and pressure to ensure the material is fully consolidated. Trapped air pockets act as insulators, preventing heat dissipation and causing localized soft spots internally.
• No Forced External Heat (Initially): Do not apply direct, high external heat (like a heat lamp or gun) immediately after application to accelerate the cure. This will only speed up the surface reaction and exacerbate the difference in cure rates between the skin and the soft core. If post-curing with heat is required, wait until the putty is already firmly set throughout and follow a slow, controlled temperature ramp.