There is several products out there and I've used simple roll on 2 part systems from NAPA. For small holes you can lay in .016 Aluminium pieces in the initial coat to make it stick and give it a second/third coat by brush to seal. They ain't coming out. Not the cheapest way to seal, but better than most paints or primers. Prep work, i.e. wire brush, sanding, etching, final wipe are key!
Best regards, Willy T.