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http://www.fixmyroof.co.uk/remove-sil...

This is what happened to me when I chose to use WD40 to remove old bathroom silicone, rather than my usual method of just plain elbow grease and cutting it out. I then normally clean it up with Isopropyl alcohol or PVCu frame cleaner.

Unless you can get 100% of the old WD-40 off of all mating surfaces your run the risk of what happened to me, contamination. This took about two weeks to happen but it slowly spread until a complete re sealing of the bath is now required.