[Dennis]

If there is black rubber on your portholes because the heat has melted the gasket on to the porthole window you need to clean the rubber off the window. If you see that the rubber gasket has micro cracks and look as if it is deteriorating you can put some black silicone over the gasket, smooth it on and let dry for 24 hours before closing. Ultimately a new porthole is the best repair for a gasket that has seen better days.