Seam sealer on underside?

I use a homemade waxoyl on my old jeeps. They are notorious for frame rot. Just make sure all your welding is done before you do it. (IE frame is completely finished). The home brew recipe I’ve used on multiple vehicles is:
Large old pot
Jug of used motor oil
Toilet bowl wax rings

I use the side burner on my barbecue
Heat oil
Break up wax rings
Mix until melted
Get the consistency you are looking for. If you want thicker, use more wax
Use whatever tool to get wax where you need it (turkey baster, garden sprayer)
It will turn into a frosting type coating depending on the wax ratio

Just putting this out there as good for thought. On hot southern days my frame will weep a little bit of waxy oil