Substrate shows through. Old paintwork or body filler spots: are not hidden by the topcoat.
Causes
1. Color of substrate not matched
2. Color with poor hiding power (e.g., no heavy metal pigment)
3. Topcoat overthinned
4. Topcoat not stirred properly prior to use
5. Paint layers applied too thinly
How to avoid
1. For translucent colors in particular (pearl effects in a three-layer system), a uniform substrate is required.
2. For colors with poor hiding power, use Tintable Filler. Alternatively, use primer filler/surfacer mixed at a level of brightness matching the topcoat, such as Primer Filler black mixed with Primer Filler white, or, with the wet-on-wet method, Non-Sanding Filler dark grey mixed with Non-Sanding Filler white.
3. Stir the paint material thoroughly and thin only as specified.
4. Spray sufficiently thick coats and allow for sufficient flash-off times between coats (solid colors 50-70 µm, metallic effects 15-25 µm).
Repair
After drying, sand down and apply a new finish.