Why is it important to use primers when solvent welding PVC?

Using primers when solvent welding PVC is crucial because they play a significant role in preparing the surfaces to be joined, ultimately creating a stronger and more reliable bond. Primers work by softening the PVC material at the joint surface, which increases the surface area for the solvent cement to penetrate. This softening process allows the solvent to dissolve the layers of PVC, effectively fusing the two pieces together as the solvent evaporates and the materials harden again. The enhanced penetration and fusion of the materials lead to a stronger joint that can withstand various stresses and pressures experienced in a plumbing system.

In contrast, the other options do not accurately capture the primary function of primers. For example, while larger pipes may have specific requirements, all sizes benefit from the use of primers. Primers do not act as a water seal; that is the role of the solvent cement itself once cured. Additionally, while mold prevention is important in some plumbing contexts, it is not a function attributed to primers in the solvent welding process.