I have a rusty engine mounting, the cause of which has been eradicated. I have cleaned the mounting up. Can I spray it liberally with WD 40 to protect it or will this cause the rubber to perish?
WD 40 according to Wikipedia is a light oil in white spirit. The oil could affect the rubber but the amount deposited from a light spray is unlikely to cause any major problem.
WD 40 is unlikely to do a great deal of good either. IMHO it would be better to degrease, remove as much rust as possible by chipping and scraping followed buy several applications of a proprietary chemical rust remover then paint with a couple of coats of Hammerite.
IMHO if it is difficult to reach a spray of Waxoyl does the business and it is safe with rubber- being lanolin based. I have heard WD40 can make rusting worse.
ACF-50 or corossion block is what you need. Search for it, You'll find its more easily available in Motorcycle shops as the chandleries have been a bit slow to catch on. I've used both for the last 5 or 6 years. Fantastic stuff
Read about it here http://www.acf-50.co.uk/marine.htm