cleaning dirt out of the chasis rails

I've rescently purchased my first Landy and it clearly has been used off road probably in a swamp by the looks of things. I have been preparing it for winter (i live in norway) so doing all the usuall mechanical servicing and as I've just got it I've been poking around underneath. I have pressure washed it several times and of course have found some rust, which I've spent the past week welding up, sticking patches on....time consuming but I have learnt to use a mig welder But I have noticed there is allot of dirt and rust inside the chasis rails. Where I have welded I have used air and the vacumn cleaner to try and suck and blast dirt out, but does anyone have any good ideas to remove mud and rust from inside the chasis rails/box. Can I just blast water in there, is this a good idea as it may trap some water? I guess the chasis must fill up when you wade in deep water so theoretically should be ok to stick the pressure washer in there? Any ideas, also after Ive cleaned it whats best should I stick waxoil in the chasis to try and prevent any rust getting worse? The car is 18 years old but it was allot of moeny for me and I want to keep it for along time so I want to look after it, thanks for tips Jim

Dear Jim...please use the search button, All the information that you need is within this forum. Look for the SEARCH button and when you press that.. type in what you are looking for....Trust me...the SEARCH button is magic..