4WD Rust Removal

Hello all,

Purchased a 2004 Nissan Patrol privately for my wife recently & while I'm no mechanic it looked fine. Was provided with ample paperwork suggesting the car was roadworthy.

Have since been advised the undercarriage has what appears to be superficial rust & has been sprayed with a black solution, either to hide the rust or treat it, which I'm not sure at this stage.

The car was in the UK for 3 years & apparently they use a salt solution over there to clear roads of snow & ice during winter, hence the rust.

The mechanic who performed the roadworthy in Melbourne appears to think that the rust isn't an issue with regards to the RWC he issued three months ago & is therefore not his problem. Am learning very quickly that this type of thing is very "subjective" in that from one mechanic to the next, they have varied & differing views on what constitutes a roadworthy car & what doesn't.

So I'm after some advice in terms of what products or steps I should take with respect to removing the rust & having it treated. Naturally, if I leave things they'll only get worse. I live in North-East Victoria so my options with respect to where I take this car are limited. I would imagine specialist 4WD businesses would have seen this problem & know what to do but they are few & far between in these parts.