Whether it’s a remedial (rust) repair, full body repaint or crash repair there is one small detail that many bodyshops – even windscreen installers – are overlooking.
If there are remnants of the old Polyurethane adhesive they must either be masked before painting or removed completely. Painting over cured urethane is not recommended as paint – or primer – does not adhere to it. From the moment fresh PUR is applied to it, it becomes a floating bit of paint ‘skin’ and will not form a strong bond once it cures. Urethane however, will stick to cured paint (provided it has been allowed a minimum of 24 hours after bake).
Before bonding to a freshly painted surface the ‘painted over urethane’ needs to be cut back to provide a suitable substrate. Any bare metal exposed must also be touched in with an adhesion promoter. For new, painted surfaces (ie where there is no old PUR) a line of primer is recommended by most manufacturers however it is more a belt-and-braces approach in a fast-fit environment for primer-less systems.
It is also important to allow enough time for the paint itself to cure. If the painting has been done in an oven, at least 24 hours is required after bake (to cool down). The longer it is left, the better.