Often paint is applied over a previous colour coat, in the case of decorative paint to change the colour or to restore the original colour and gloss, due to fading, weathering or exposure to staining sources, e.g. tobacco or coal smoke.

Where paint is to be applied over a previous coat, there are two important questions:

1) Is the original coat in sound condition?

2) Would applying another coat lead to excessive paint film thickness.

Because is rare for someone to want an article painted if there is nothing wrong with the original finish, faults like cracking, chipping, blistering must be viewed very cautiously and if in doubt, remove the paint layers until a satisfactory surface is found. Burying faults only reoccur later on and cost you twice as much to put right.

Film thickness can be more difficult. Paint is designed to work at an optimum film thickness e.g. 1.5-2.5 thou (37-62 microns) per coat with primer undercoat and topcoat. H.5-7 5 thou total film. As paint film thickness increases, less of the characteristics of the substrate and more of the characteristics of the paint are seen, i.e. the paint behaves less like the metal, wooden or plastic substrate and more like the dried up remains left in the bottom of the paint can. It is more likely to crack, will remain softer and more likely to chip or peel. Paint film thicknesses of over 12 thou must therefore be regarded as suspect.

The type of original finish must also be considered and repairing like with like is liable to lead to less problems.

Identification of original finish is fairly easy. Lacquer dry systems (nitro cellulose and thermo-plastic acrylic) are detected with a cellulose thinner rub. The finish melts readily, making a sticky mess and lacquer systems clog orbital sanding machines.

Original enamel (high bake) two pack and fully cured synthetic systems should show no solubility to cellulose thinners, though often synthetic finishes show some colour loss and some softening, which can be detected by a characteristic smell when orbital sanding.

Basecoat and clear systems can be detected by polishing with compound. If no colour is removed the system has been clearcoated.