Shrinkage during cure is a common and often unavoidable characteristic of many thermosetting polymers, including two-part epoxy and polyester repair putties. As the chemical reaction completes, the material densifies, causing a slight volume reduction. This shrinkage concentrates stress and can manifest as gaps, low spots, or hairline cracks at the edges of the repair, compromising the seal and structural integrity.

Here are genuine, non-product-specific solutions to minimize and manage the effects of cure shrinkage.

1. Preparation to Contain and Mask Shrinkage

Proper preparation can force the putty to shrink internally rather than pulling away from the edges.

• Mechanical Keying is Paramount: The deep V-grooving or chamfering of the defect (as detailed previously) is the first line of defense. When the putty cures and shrinks, a strong mechanical lock prevents it from pulling away from the anchor points. Instead, the shrinkage stress is distributed throughout the bulk of the material, not just at the surface bond line.
• Controlling Edge Geometry: When abrading the surface, ensure the perimeter of the prepared area is slightly undercut where possible. This is a subtle technique where the edge of the repair area forms a slight inverse taper. As the putty shrinks, it pulls into this undercut, tightening the seal rather than lifting away from a flat or convex edge.
• Slight Overfill/Overbuild: Intentionally overfill the repair area slightly above the required final height (known as ‘overbuild’). Since most shrinkage occurs perpendicular to the surface (loss of volume), the bulk of the material will settle. This leaves a margin of material to be sanded or ground back to the correct profile, ensuring there are no low spots or gaps at the final finished surface level.

2. Application and Curing Techniques

The way the putty is applied and cured affects the final volume stability.

• Layering with Successive Overfill: When building up a thick repair using multiple layers (to manage exotherm and thickness limits), slightly overfill each layer. The minor shrinkage in the lower layer will be compensated for by the fresh material of the next layer.
  • Tip: Wait until the previous layer is fully cured (or at least solid enough) before applying the next. This prevents cumulative shrinkage that would occur if all the material cured simultaneously.
• Tamping/Compression: After applying the putty into the prepared defect, firmly compress or tamp the material into the cavity using a clean, rounded tool or spatula. This action ensures the material is fully forced into all crevices, eliminates trapped air bubbles, and achieves maximum density before the chemical reaction sets in. The denser the material is before cure, the less volume change (shrinkage) may occur during the cure.
• Avoid Excessive Heat: While heat can speed up the cure, curing too quickly, especially with forced heat, can increase the rate and degree of shrinkage as volatile components are driven off too rapidly. Cure the putty at the manufacturer’s recommended ambient temperature, allowing for a slower, more complete, and less stressed cure.

3. Post-Cure Management (Finishing)

If minor shrinkage still results in small gaps or low spots, a final step can rectify the appearance and integrity.

• Final Skim Coat: Once the primary repair has fully cured and any major grinding is complete, apply a very thin final layer (a “skim coat”) of the same putty to the entire repaired area, focusing on filling any minor low spots or perimeter gaps created by the initial shrinkage. Because this coat is thin, its own shrinkage is minimal, and it adheres strongly to the fully roughened and cured base material.
• Sanding/Grinding: The overbuilt material (from the overfill step) should only be sanded or ground to the final profile after the full cure time specified by the manufacturer. Finishing too early, while the putty is still curing and shrinking, will result in a sunken or low area once the shrinkage completes.