to strip underseal of (depending on how messy you want to get) you can use a decent paint stripper (think nitromors but not as agressive) and then for actual rust strip i use a variety of wire brushes.
Angle grinder attachement knot brushes are best for large areas but are quite violent.
For the hard to reach places i use drill attachement wire brushes.
Then for the really really hard to reach places Bilt Hamber Deox gell.
becarefull when stripping off rust as you will always be left with a small amount of carbon (little black dots) that look harmless, they will make the rust come through again if not removed. They are super hard aswell so will take some effort.