Substrate

Mapping

The edge of a previous repair or filler is visible through the top coat — the outline of the repaired area shows through on the surface as a subtle shadow or step. Sometimes sanding scratches that were not properly feathered out are also visible.

Cause

Spray filler applied to an unsuitable substrate — differences in surface tension cause the edges to show through
Substrate sanded too smooth before applying the filler — the edges crumbled during sanding, leaving an uneven, ragged surface
Repair area not properly feathered into the original paint system
Filled area not sanded to a sufficiently smooth finish
Incorrect sanding technique or poor sanding equipment
Spray filler applied over the top coat of the old system instead of directly onto the metal
1K filler primer used for larger dents — shrinkage leads to edges showing through
Poor or insufficient filler adhesion due to inadequate degreasing
Application of paint products onto soft finishes such as thermoplastic acrylics or insufficiently cured OEM systems

Prevention

Carry out a solvent test before sanding — use the recommended system on soft substrates
Degrease thoroughly before sanding
Follow the correct sanding steps throughout the repair process — especially on ragged edges
Use suitable sanding blocks for smooth sanding
Check the surface of the filled area regularly during sanding
Thoroughly feather the repair area into the original paint system
Apply spray filler only to a stable substrate
Polyester filler only for filling small pinholes and scratches — not for large areas

Repair

Thoroughly sand the paint system including the problem area
Repair the problem area with a suitable system — spray filler, primer and top coat
Pay particular attention to the edges of the repaired area — a gradual transition eliminates show-through