Anytime we put new metal into a car, we use a generous amount of the Mercedes-Benz Cavity Wax, it's like a beeswax substance. Works great in sealing off joins, etc..

We also use Wurth metal prep and weld through primer. MB also does sell their underseal in big cans, we will use this on the underside of a floorpan, then put the cavity wax over it.

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Regards,
Aaron