Salt on the roads.

[Reckless Rat]

If you're tight, like me and some others on here (no names no pack drill) you can make your own "waxyoil". All you need is a few white candles (6) some white spirit and some engine oil (used if you like it matters not) Stick the candles in the freezer and then when good & hard give them a thorough bashing with a lump hammer. Put all the bits (the smaller the better) in a bucket, removing the wicks. Add a bottle of white spirit, cover and leave to mature under the bench for a few weeks. When the candles have dissolved add a similar amount of engine oil to the mix and you're good to go. If the candles haven't fully dissolved you can (surreptitiously) borrow the kitchen hand blender to help them on their way but don't get caught. I did and I couldn't walk properly for a few days.

Paint it on all the underbody. It will keep out salt & humidity.

Or spend £50 and buy a 5 litre can of proprietory underbody sealant with a similar trade name that will do exactly the same thing.