The nitromethane portion of rc car nitro fuel is actually not overly hygroscopic, which is the term for a subsctance that readily attracts and retains water. The hygroscopic portion of nitro fuel is the methanol rather than the nitro.

Rust in nitro engines is caused by water moisture drawn in from the air by the hygroscopic methanol in the fuel and accelerated by any residual acids left in the engine.

Cheaper methanol has residual acids in its natural state that greatly enhance rust. Good quality fuels use good high grade methanol with very low acid contents.

The second source of residual acids is from partially burned fuel. This is the most common source of the acids which together with the moisture can cause the crankshaft and bearings to rust.

Most people think you should run an engine dry after you run it but I strongly advocate against this for the reason mentioned above. It leaves more half-burned fuel in the engine than otherwise. Also running the engine bone dry puts undue stress on the engine parts up until it runs completely out and is just as bad as running the engine too lean. And if you use a good quality fuel with the lubrication properties of a good oil package, then you will actually experience less rust than if you burned all of the fuel out and let the engine sit.

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