Ye gods! With my imagination what a picture — the stuff of nightmares or what?!!!
Black saddle – hot sun – no trousers? Now that’d make you dance! (Black Bottom – dance? Aw! Forget it! ) Kinda makes you glad we don’t have too many hot summers though!
What about cling film for a quick fix? Longer term what about a fitted cover? Frankly I can’t think of a reliable, durable and flexible coating you could apply to seal it. I hesitate to suggest clear boot polish as it just might react with the rubber but it might help.
If it is as bad as it seems then it would appear that the rubber was badly cured or made from defective materials. But then again I favour Barbour gear so I too leave a trail that is both black and greasy!