Go over the area to be painted with white spirit and a rag to remove all traces of grease and adhesive.

It might look perfectly clean and ready to paint, but even the smoothest surface to the naked eye is not flat and the microscopic pits will harbour grease and the remains of sticker adhesive. The white spirit will act as a gentle solvent and the grease/adhesive will dissolve in it and come away on the rag.

I would suggest repeating this process a couple of times until there is no grease left. The remaining white spirit will evaporate. It will not harm you paintwork or plastics - just try and avoid long contact with rubber components.

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