A product called Dinitrol grade 3125HS I've found superb over the years
One great advantage is it comes in 500ml aerosol & can be applied directly to rust& dust
Also it says it won't damage rubber seals etc
It appears to boil/foam it's way on
It says on the tin:-
High Solids corrosion prevention for cavities and open surfaces.
Brown,water displaceing, dry, waxy film
Its also available in canister form for use with a compressor & shutz type gun
The firm who sell it are Frost auto restoration techniques Ltd Crawford Street Rochdale 01706 658619
The last 500Ml Aerosol I bought cost £15.25
Not cheap I know
The canisters I think are cheaper
However very handy to have when I go under Miggy doing other stuff
Another tip I've found to be good is near me is a garage with a drive thro car wash which has a underneath wash option