Ignore anything aerosol-based as it will not last as long.

I find the best thing to do first is to take back all the old and loose stuff and use the types of stone-chip that you pour into a gun and spray on (dedicated type of gun). It should go on a bit like a web-effect and is basically what we use in the trade on the bottom of doors and sills before painting over. It might cost you more initially, but the benefits far outweigh the money spent.

Once that is on, then you can apply a brush-on type of underseal, should you need it, or as I have done in the past, just add another couple of layers of the stone-chip. I did this once on a car about 4 years ago, and the underside is still as the day I did it as far as longevity stands. IT also gives a better-looking finish.

Any local paint and body supply shop should be able to provide you with the equipment and supplies. The stone chip we use in my bodyshop costs us a shade over £5 per litre, and 3-4 of those should do you well over the whole underneath should you take it that far.