I like to brush primer on areas like this to work it down into the cracks. I use PPG epoxy primer, but you can use Rustoleum "rusty metal primer" in a spray can. Load it up, brush it in, & let it dry. Get the autobody seam sealer in a caulking tube: you can use a towel to brush over it to look factory if you like, or just smooth it out with your fingers. After it drys prime the area again. Make sure you do both sides to prevent rust.
aka "Tha Driver"Custom Fiberglass Parts