Which surface property is essential for mixing plastic filler?

• A. Porous
• B. Non-porous
• C. Rusty
• D. Glassy

Answer: (B)

Non-porous surfaces are essential for plastic filler because this resin-based material needs to wet and bond to the substrate. A non-porous surface stays clean and sealed, so the filler doesn’t soak in or lose solvents during mixing and curing. If the surface is porous, the resin can be absorbed into the pores, leading to weak adhesion, pinholes, and an uneven finish. Rusty metal is usually porous and contaminated, so it must be treated and prepared to present a clean, non-porous base before applying filler. A slick, glassy surface can be compatible as long as it’s clean and non-porous, but the key requirement is preventing absorption to ensure proper bonding and curing.