I got a new one for mine was getting rather shitty, cost me £80. To treat the underside first you need to get all the muck off. Use WD40 or petrol to remove all the road grime and then use a wire brush attachment for an electric drill to remove the surface rust.
Treat any rust then apply underseal or black waxoil, It’s also worth filling any box sections with waxoil while you’re under there