You'll need to prep the areas in question by using a red scotch just to give it a key. Etch prime the bare metal first. There are many sealant products out there, I personally like teroson adhesive sealer for older cars and teroson 6in1 for modern ones. Once you've sealed up all the seams you'll then need to spray underseal or stonechip on the areas in questions or the whole underside. Underseal stays tacky and can't be painted over, stonechip dries and can be painted over. Once that stage is done and dry you'll then need to spray cavity wax inside all the chassis rails, sills etc, buy the proper wax spray gun with the long nozzle. If you end up buying waxoyl make sure you warm it up first before applying, get it nice and runny and it'll flow into all the crevasses and set, protecting the metalwork. Don't bother with aerosol cavity wax. You can also spray the cavity wax over the stonechip or underseal if you wanted to. I restore classic cars for a living, I Hope this will put you in the right direction.