Why do some manufacturers advise against sanding the polyester-based putty with water?

Manufacturers often advise against sanding polyester-based putty with water primarily because the putty tends to absorb moisture. When water is introduced during the sanding process, it can penetrate the putty, which may compromise its integrity. Polyester-based products are designed to set through a chemical curing process, and the introduction of moisture can disrupt this process, potentially leading to issues such as reduced adhesion, increased shrinkage, or even soft spots. Additionally, moisture may hinder the putty's ability to harden properly, which can result in an uneven or compromised surface after sanding.

Other approaches, such as dry sanding or using appropriate sanding methods that avoid moisture, are recommended to ensure the putty maintains its intended consistency and performance. Understanding the material properties and the correct sanding techniques is crucial for achieving a high-quality finish in refinishing work.