Help with gutter spout caulk.

[Brewhedd]

I need to recaulk my gutter spouts. The caulk dries hard with no flex and has great adhesion. I believe the same product is used around tail and marker lights. I can't find or figure out what this is. It is NOT a Geocell product. If you don't know what it is please don't provide information or suggestions as I have already purchased a Geocell product that didn't work. I also called them and they don't have a product that is non flexible.