I have used Permatex Ultra Black RTV silicone gasket maker for a number of different projects. It's pretty tough, but it is black. I've sealed around my windshield, sealed around valve cover bolts, even used it to fill in holes make by rocks in the headlight lenses of my old '91 Camry. Regular household silicone wouldn't last long but this stuff seems to be really strong. I have filled in 2 pretty good sized holes in the headlight lenses and they've stood up to 3 winters now. One hole was big enough to put quarters in. I had to layer in the silicone over a couple of days.
The only down side I've seen is that it is black and will get on everything if you aren't careful, but once cured, it stays put.
Just throwing that out there