Auto Body Finish Techniques

Car exteriors are damaged by accidents and impacts with other objects, but new techniques and refinishing methods can restore the vehicle. Once damage to the metal exterior has been repaired, the finish work must be done. Auto refinishing is a process that requires a series of undercoats before the final color coat of paint. Often called primers, sealers, surfacers or adhesion promoters, undercoats are applied to prepare the car for color paint. Most are clear sprays.

Removing the Dent

Dents must be removed and repaied.

The three most commonly used tools to restore a car's sheet metal configuration are hammers, dollies and spoons. Stud dent pullers are used in difficult-to-reach spots.

Filling, Smoothing and Sanding the Repaired Area

Different types of filler are used in dent repair.

The most common filler is plastic or polyester, but lead has been used for generations. A solid bar of lead is melted to make a thick paste that is molded to fill the dent. Then it's shaped and smoothed. Products containing resin have better solvent and corrosion resistance and stabilize quickly for sanding.

Surface Preparation

Undercoat aplication must be done after the metal surface has been repaired.

Before paint can be applied, cleansers and etchers are used to smooth the repaired metal surface. Several different products are used over anti-corrosive primers to create a surface to which the paint will adhere smoothly. Undercoats are adhesion promoters whose sole purpose is to ensure subsequent layers will adhere to the vehicle's metal surface.

Anti-Corrosion Primers

Primers are applied to prevent corrosion and smooth the repaired surface.

There are several types of primer that are used to prevent corrosion by protecting the substrate of the car. One is 2K Epoxy, which effectively refinishes bare metal and has adhesion and sealing properties. Its disadvantages are that it doesn't seal well in cold weather and it's difficult to sand. Self-etching primer provides good corrosion resistance and dries quickly. It's used extensively in auto-repair shops. Enamel primer cures by oxidation from air; it is corrosion resistant and inexpensive.

Primer Surfacers

Primer coats cover chips and scratches as well as dents.

The purpose of a primer surfacer is to provide a smooth surface by covering scratches, gouges, chips and small dents. Primer surfacers are used over anti-corrosive primers, and they must be sanded to smoothness. A two-component primer like 2K Polyurethane Primer Surfacer fills well and sands easily. Made of durable urethane, it's commonly used in auto repair. Polyester primer is often called "sprayable bondo." It has a low solvent content but is less flexible than other urethanes. Lacquer primer dries by evaporation of its solvents, and it's not as durable as urethane primers. Lacquer primer is easy to sand.

Primary Sealer

The final step is applying the color coat of paint.

This is the last step in preparing the vehicle's surface for the color coat of paint. It makes a smooth non-porous foundation and, because of its low viscosity, goes on smoothly. The top choice for auto painting is durable 2K Polyurethane Primer Sealer, which rarely needs recoating. The 2K Epoxy sealer can double as a primer sealer, but it shouldn't be used under transparent or semi-transparent colors because of its instability to light. Enamel Primer Sealers are durable, dry by oxidation and have been used for generations.