100+ Free Cert III Refinishing Practice Questions

Explanation: Epoxy primer provides excellent adhesion and corrosion protection on bare steel substrates. It acts as a moisture barrier, unlike high-build acrylic primers, which are porous and can trap moisture. Basecoats do not offer corrosion protection and require a primer/sealer underneath.

Explanation: Wax and grease remover must be used before sanding to strip surface oil, road tar, and silicone. If you sand first, these contaminants will be ground deep into the sanding scratches, making them extremely difficult to remove and potentially causing paint defects like fish eyes. It does not etch metal or act as a guide coat.

Explanation: The standard trade rule is to skip no more than 100 grit sizes (e.g., P180 to P280, or P240 to P320). Skipping too many grits fails to completely level out the deep scratches from the coarser grit. Over time, the topcoat solvents will cause the primer or paint to shrink into these deep scratches, making them visible.

Explanation: Aluminum is highly susceptible to galvanic corrosion. Tools, sandpaper, or Scotch-Brite pads that have been used on steel must never be used on aluminum, as they deposit tiny steel particles that initiate galvanic reactions. Wire brushes are too aggressive and scratch aluminum excessively, strong bases corrode it, and body filler needs a primed surface for reliable long-term adhesion.

Explanation: Polypropylene (PP) is a non-polar plastic with low surface energy, meaning paint will slide right off it. After cleaning and scuffing with a fine pad, a plastic adhesion promoter (usually chlorinated polyolefin based) must be applied to chemically bond the plastic and the primer. Acid etching does not work on PP, coarse 80-grit sandpaper destroys the plastic texture, and standard metal epoxy primer will quickly peel off.

Explanation: A float test is a reliable shop method for identifying plastic types. Polyolefins like Polypropylene (PP) and Polyethylene (PE) have densities below 1.0 g/cm³ and float in water, indicating they require a plastic adhesion promoter. Non-polyolefins (such as ABS, PC, or PUR) have densities greater than 1.0 g/cm³, sink in water, and can often be painted without a specialty adhesion promoter.

Explanation: Polyester body fillers are sponge-like and porous. If they are wet-sanded, they absorb and trap moisture against the underlying bare steel, which leads to rust blisters forming under the paint later. They do not dissolve in water or cure faster with water, and wet sanding reduces rather than increases static electricity.

Explanation: A guide coat (either a dry powder or a light mist of contrasting aerosol paint) is applied over cured primer surfacer. As the technician block-sands, the guide coat is removed from high areas and remains in low areas, scratches, or pinholes, visually indicating where the panel needs more leveling. It has no chemical sealing, lubricating, or anti-corrosive properties.

Explanation: Feather-edging is the process of sanding down the hard edges of broken paint layers (clearcoat, basecoat, primer, bare metal) into a gradual, stepped taper. This creates a completely flat, transition-free slope so that when new primer is applied, there is no ridge or line where the old paint ended. It has nothing to do with dusting tools, paint mist patterns, or fuzzy plastic textures.

Explanation: Most commercial rust converters contain phosphoric acid. The phosphoric acid chemically reacts with iron oxide (rust) to convert it into iron phosphate, a stable black coating that resists further corrosion. Hydrochloric acid is too aggressive and leaves corrosive chloride residues, sodium hydroxide is an alkaline paint stripper, and isopropyl alcohol is a degreasing solvent.