chassis rust

I have a D40 STX 550 Spanish built, never off the bitumen.
Used a borescope ($200 from TradeTools) and found I have rust establishing inside the chassis rails. The exterior paint of the rails is fine with no rust at all.
A rust guy said don't use fishoil or lanotec as dust sticks to it making it impossible to manage later. Also said his own propriety underbody products are too thick to spray inside.
Any thoughts?

I might go check mine, it's at least two years older and it's seen a LOT more action!

Bash the crap out of it with a hammer..mainly around the affected areas (anything chunky and flakey)..hose out chassis...flood that sucker with a garden hose from both sides and get all the dust and any rust flakes out...let it dry and get some of the por15 (google for supplyers) metal prep.its a nasty chemical that is suppose to kill rust...get a body shutz gun with a 360°nozel...(cheap one will do)fill it with metal prep and squirt away.let that take hold for 30mins and hose out..(dont do it on the lawn)..take the same body schutz gun once cleaned and fill it with either por15 paint or somthing else similar and give it another coat of paint.
Por15 products are quite nice..not sure on the longevity but they are rated highly.it will cost probably $200 to treat the chassis rails including the gun.
Never looked in mine but id imagine they are not painted at all....not sure i want too!

Someone suggested Penetrol. Thin solution, gets everywhere, dries hard after a few weeks & stops rust. Anyone had experience with this product?

It might stunt its growth...most decent products will need your car off the road for probably 2 days to dry properly..the last thing i penetroled hasnt got any worse but...penetrol is very soft and will probably split and get rubbed away with a bit of sand and mud.i penetrol some of my machinary..its almost silicon like

The metal prep sounds like a rust remover (acid). I may have to do that, just a bit concerned where the chassis metal folds over another layer. Any residual acid may cause corrosion. I guess its all about washing it out the metal prep with heaps and heaps of water.

Its acid but not very long life acid.you could leave it on for weeks and it would just dry out ..add a good flushing with water and its disabled..lots of water tho!! Metal will turn black and chalky once dry...some great youtube vids on it

It may have salt in it from the trip over from Spain which will make it harder to stop.
My concern is that I will not be able to stop the corrosion.

Whats that shit the yanks spray all their underbody and seams with up north where they salt the snow?

Even if you did nothing.if you kept it clean in there like hosed out every few months it would get a rust coat and not much more..think of farm machinary..sure the clean bits are rusty but they are still there unlike the parts sitting in the dirt they are starting to dissapear

The POR15 does look like a good system. It does warn about other metal prep products that leave a harmful residue. It does look good and once overcoated should be long lasting.

Por15 also comes with a big health warning for anyone wanting to use it...im pretty sure its not for sale in the state of california!
A lot of other products are phosphoric acid based..these are mainly metal cleaners but will remove light surface rust...there is also a brand called permatext and K&H do a milk like product..in the past i have used both..must say i never loved them but they have been available for 20+years.sold as rust converter..but the por15 is the best one ive used..each step you can see its getting cleaned and treated.not just masked like the other converters seem to do.

I'm 63 so nothing can get me now that causes problems in the long term.
Thanks for the feedback, looks like my best bet.

Thats the spirit!!

But wear a mask..u could end up with a nasty cough for a couple of hours.

A good description:
http://www.ppcco.com.au/restoration.htm

I bought some 'cavity wax' for my last 4wd, seemed to work well, not sure whether dust will stick to it though.

How about something like this to clean/rinse inside the chassis tube:
http://www.youtube.com/watch?v=VAEl294mhk0
Bunnings sell an "expanded drain cleaner" for $50.00 includes 15 M of hose and the jet.
This is rated at 250 bar (3,625 psi).

And to spray the chemicals/paint around we have:
http://www.kbs-coatings.com/spray-guns.html
An underseal spray gun with optional 36" flexible tube.
Discussion forums confirm that it will spray the por15 paint but may have to dilute 10%
The body Schultz gun seems unsuitable for por15 paint.

A body schutz gun will spray just about anything! But use half to quarter throttle if you spraying a water like substance.but yes that nozzle is just about perfect..as you will want to feed it down the rails as far as possible and spray as you draw back.only way to get a consistant coat..not sure if the rails are hollow from front to back...might have the test with water first if you want a perfect job.and it sounds like you do.i would say some sort of test bunny needs to be found to check the metal prep and paint consistancys while spraying...not much por15 paint is required for the job.1-2 ltrs does a full floor pan on a old holden sedan

It could have just been a viscosity issue that the body Schultz user had, perhaps it does work?
I have examined the chassis tube with a snake camera almost the full length and no baffles - full hollow.