How To Troubleshoot Automotive Paint: Causes and Cures

Automotive Paint Troubleshooting Causes and Cures
 

Through the years the du Pont organization has met and solved practically every automotive refinishing problem that can be imagined. In the interests of better refinishing du Pont has made the following information available.

Shrinking and splitting of primer-surfacer

Cause:

  • Improper cleaning of the surface.
  • Improper “feather-edging” broken area.
  • “Piling on” in heavy coats.
  • Insufficient drying between coats.
  • Fanning air to force drying.
  • Applying on a cold surface.

Cure:

  • Use clean rag wet with Prep-Sol to remove wax, polish, and grease. Where bare metal is exposed, sand bright and treat with a metal conditioner.
  • “Feather-edge” old finish about 2″ to 4″ back from the edge of the spot, finishing up with No. 400 sandpaper to avoid deep scratches.
  • Apply in medium wet coats.
  • Allow solvents to “flash” off before applying succeeding coats. Let last coat dry 30 minutes before sanding.
  • Do not fan air to force drying — it causes surface drying and traps thinner which penetrates scratches in the old finish.
  • In cold weather, make sure the metal, the primer-surfacer, and the shop are approximately the same temperature.

Overspray—Dry spray

This condition is usually found where the finish forms a centerline on cowls and turret tops. If the left side of the car is painted first, the overspray appears to the left of the centerline when the right side is sprayed—or vice versa. At this point, the gun is held at an angle and farther from the surface than at other points of the stroke.

To overcome, spray side of the car on which overspray appears with a wet mist coat. A mist coat is made by adding several parts of thinner to the color left in the cup, or it may be a straight coat of a good thinner.

Orange peel

Cause:

  • Improper atomization.
  • Sprayed improperly.

Cure:

  • Use recommended air pressures at the gun and follow thinning instructions.
  • Use arm in full, long strokes, paralleling surface with as little wrist action as possible. Hold the gun at a right angle to and from 6 to 10 inches from the surface.

Rust under film

Caused by the presence of rust on the surface before finishing—and no paint will adhere to a rusty surface. To avoid this, always sand all rust spots bright and treat the bare metal with a metal conditioner such as “Deoxidine,” “Metalprep,” etc., before finishing.

Wash with water and dry thoroughly. Always prime the surface immediately after cleaning, as rust will set in if the bare metal is exposed too long.

Wrinkling of Dulux

Cause:

  • Application of an excessive number of coats improperly dried between coats.
  • Overloading of one or more coats.
  • The subjection of air dry Dulux to elevated force dry temperatures without first adding Dulux Baking Reducer.

Cure:

Always follow recommended procedures when reducing and applying successive coats of Dulux, and always add RK-5756 to 93 Line if the film is to be force dried.

Finger prints showing

This blistered outline of fingerprints shows what can happen when bare hands touch any surface about to be painted. No matter how clean the hands may seem, there is almost always some dirt, grease, oil, or perspiration which will cause blistering, rust, and poor adhesion. Keep bare hands off the surface!

In automobile plants, bodies are never directly touched with human hands after being chemically cleaned—the operators wear cotton gloves. It’s a good practice for ALL painters to follow!

Runs or sags

Cause:

  • Too much thinner.
  • Too heavy coats.
  • Too wet coats.

Cure:

  • Reduce material according to label directions.
  • Regulate fluid adjustment on the spray gun (turn to the right) to cut down the flow of material.
  • Do not hold the gun too close.

Pinholing in lacquers

Cause:

  • Moisture in spray lines.
  • Trapped solvents.
  • Insufficient atomization or breaking up of material.

Cure:

  • The drain valve of the air transformer should be opened daily to allow drainage of collected moisture. Remove and clean the internal cleaning section of the older type transformers at regular intervals.
  • Do not apply materials too heavily but spray in uniform, normal coats to allow proper evaporation of solvents. Use recommended thinners.
  • Increase the pressure of the gun for proper atomization.

Poor adhesion

Cause:

  • Improper preparation of the surface.
  • Wrong undercoats.

Cure:

  • Thoroughly clean the old finish with Prep-Sol to remove grease, wax, polish, and other foreign matter. Improper treatment of bare metal surfaces also causes poor adhesion.
  • Follow “Procedures” and label directions.

Chalking of lacquers

The left section of this panel, which was exposed on a paint farm, shows (in comparison with an unexposed section on right) what happens when a finish chalks. A natural failure, chalking is the gradual breaking up of the film under weathering and exposure to the sun’s rays. It results in a gradual loss of gloss and powdering of the surface.

When this condition is encountered, rub and polish the surface to remove “dead” pigments and get to the “live” film beneath. Then wax the finish to protect and prolong its life. The use of a mist coat mixed with a slower-drying thinner on a finishing job will enable the film to set better and aid in retarding chalking.

Rough, dirty finish

Cause:

  • Applying finish over dusty surface.
  • Dirty and dusty shop or spray booth conditions.

Cure:

  • “Tack-wipe” the surface immediately before spraying color coats in order to remove dust and dirt. Tack rags were a regular part of finishing equipment in the old days when varnish was used. Now with synthetics, it’s a habit to be cultivated again. Get in the tack rag habit. They cost little—and save a lot of headaches.
  • Good housekeeping.

Lifting

This condition results from applying lacquer type products over unaged air dry synthetic finishes. It also results from applying a finish over a surface from which old wax, grease, or polish is not thoroughly removed.

To avoid the latter, always clean the old surface with Prep-Sol to remove wax, grease, polish, and other foreign matter before any sanding is done. Improper recoat time may also cause lifting.

Shrinking and splitting of putty

Cause:

Because putties usually dry quickly, they may shrink, split, and remain soft when applied too heavily, as shown by this close-up.

Cure:

Apply several light coats with a glazing knife or squeegee, allow to dry between coats.

Wet spots

Use Prep-Sol as a cleaning agent to remove wax, grease, and polish from the old surface.

Crazing, cracking, and checking

Cause:
These conditions, each a degree of the other, result when:

  • Topcoats are applied before the undercoats are dry.
  • Too heavy coats are applied, resulting in non-uniform drying throughout the film.
  • The new finish is applied over excessively built-up, aged. and cracked old finish.

Cure:

  • Always let undercoats dry thoroughly before spraying color coats. Weather conditions will alter drying time—so do not follow a standard time.
  • Do not “pile” on coats.
  • If the old finish is made up of an excessive number of coats and/or if cracking of the old finish is in evidence, remove the old finish completely before refinishing.