In their production line paint process, most new vehicle manufacturers now use a waterborne basecoat. This layer is very solvent sensitive so exposure to products containing solvent during the refinish process generally results in unwanted swelling and ringing around the repair area.

Issues

Great care needs to be taken when sanding through the clearcoat layer to expose the waterborne basecoat during a refinish a repair. If a conventional cleaning and priming process is used the waterborne basecoat tends to react to the solvents which cause swelling of the featheredge.

Prevention

Using this recommended refinish process should avoid the potential for OE waterborne basecoat swelling.

• Sand out the defects and carefully featheredge the clearcoat.
• Wherever possible, don’t use a solvent-borne degreaser – instead, clean with a 1:1 mixture of methylated spirits and water.
• Suggestion – Infra-red the repair area for three minutes and allow to cool. This provides extra curing or hardening around the edges of the feathered waterborne basecoat/clearcoat.
• Mix an epoxy primer (such as AP-4110 Protec Epoxy Primer) as per normal. Epoxy primers contain relatively mild solvents and, therefore, aren’t as aggressive as conventional primers. Apply one medium coat and ensure the epoxy primer covers the complete repair area but is kept inside the sanding scratches. It’s very important not to apply the epoxy too wet as this can lead to a swelling of the featheredge.
• Suggestion – After flashing off, dry using infrared where possible or in cooler conditions allow an extended flash-off time.
• Prime the area with a 2K primer (such as Protec PP1550 Paraglaze Multi-prime) and stay within the epoxy primer area. Allow to flash-off and then dry using infrared where possible. 
• The repair area can then be sanded and repaired as normal, using the appropriate paint system.
• Any areas requiring spot priming should be done with an epoxy primer.
• As an added precaution, an adhesion promoter (such as AP-6000 Protec Ultra Seal) in Wet-On-Wet mode may be applied only over the repaired area prior to topcoat application.

While this process is ideal for tackling waterborne basecoat finishes, it works just as well on older ‘suspect’ finishes which may also be prone to swelling around repair areas.