Undersealing Legacy

[Lotobear]

My 09 Legacy is scabby as hell underneath - the rust is quite unbelievable for a 'modern' car.

I would like to provide some protection - what's the usual approach? Waxoyl is horrid, messy, stuff and I would like to use something else.

What is recommended - something easy to apply which goes semi hard and doesn't create a filthy mess?

I have a 2 post lift so will be doing it myself