On the WD 40 website for the multi purpose fluid there are the technical specs for its effects on various materials. Based on long periods of immersion the tech spec gives guidance on which plastics are ok and which aren’t. The problem for Joe Public is the difficulty in identifying the particular plastics that are in their equipment. Hence the general guidance to not use it on plastics. But to be honest since I first came across the WD 40 multi purpose fluid back in 1971 I have used it on parts with plastic components, including caravan door lock, without noticing any detrimental effects. My minivan continued to give sterling service after the Constables drenching of the ignition parts. But these days with more specific fluids available i tend to use them.
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