Paint reactions are usually caused by solvents in the new paint penetrating the old coating. To avoid them, it’s important to understand a little about the solvents used in common paint systems. High-order solvents such as toluene (an ingredient of cellulose thinners), have a sickly-sweet smell and evaporate quickly. Low-order solvents like white spirit have an oily smell and feel, and evaporate slowly. High-order solvents are used in cellulose, two-pack and aerosols. Low-order solvents are used in oil-based paints such as synthetic enamels.
Applying a high-order solvent to an oil-based paint that has started its curing process will inevitably cause a reaction – a severe wrinkling called ‘pickling’ that requires all the paint to be removed and reapplied. This is because the high-order solvent has the power to penetrate the oil-based coating. On the other hand, applying an oil-based paint to a cellulose or two-pack coating that has fully dried is usually perfectly safe.
The most satisfactory course of action is to use only paints of the same type – and preferably the same manufacturer. Of course, when touching-up bodywork or painting parts that don’t require a perfect finish, we often mix and match whatever paint we have in stock. Here are a few guidelines to follow. When overpainting oil-based primers and paints, only use other oil-based paints. One exception is where the paint is quite old and has fully cured (a process that can take many months). In this case, a series of very light aerosol coats can often be applied, allowing plenty of time for solvent evaporation in between. Don’t rub down the oil-based paint immediately before painting with aerosols, as this will expose a fresh surface that’s vulnerable to attack. Cellulose sprayed from a gun is likely to cause a reaction however careful you are.
Overpainting cellulose or two-pack paints with oil-based paint should be perfectly safe. If you’re overcoating a cellulose with a two-pack or vice versa, test a small area first to check for a reaction. Overpainting cellulose with cellulose, or two-pack with two-pack should present no problems. Hammered-finish gloss paints are difficult to overcoat with anything, as the finish is achieved by the addition of silicones that are likely to interfere with the finish and/or prevent reliable adhesion. If you’re not sure what type of paint you’re spraying over, experiment on a small test area. This may not always represent what will happen when the whole panel is smothered in wet paint, however. An alternative is to apply a barrier coat. This is supposed to prevent solvents penetrating the old paint beneath. It’s essential to use a barrier coat from the same manufacturer and paint system as the top coat you’re applying. It’s not always a success. You may find you have no alternative than to rub the panel back to bare metal and start again.