No, it will not rust in a vacuum. Iron needs oxygen in order to rust and there is no oxygen in a vacuum.

In space there is no atmosphere with any oxygen to react with, so any iron in space would not rust. This, however, assumes that there is really no oxygen in space. When a spacecraft is in a low orbit around Earth, there are still small traces of atmosphere it flies through.

Yes, you can spray paint over rust. Before you paint, take the time to prepare your surface properly. Spray paint surface preparation preparation is extremely important and is the best predictor of how long your new paint finish will last.

Instead of moisture and oxygen acting as the primary corrosion causes, the materials exposed to outer space are subjected to vacuum, bombardment by ultraviolet and X-rays, and high-energy charged particles (mostly electrons and protons from solar wind).

When acidic substances (including water) come in contact with metals, such as iron and/or steel, rust begins to form. Rust is the result of corroding steel after the iron (Fe) particles have been exposed to oxygen and moisture (e.g., humidity, vapor, immersion).

In space, if unprotected pieces of metal touch each other, they stick together permanently. This doesn’t happen on Earth, because the oxygen in our atmosphere forms an extremely thin film of oxidized metal on every exposed surface. In the vacuum of space, however, there is no oxidation layer.

In theory, yes – it’s an effect called ‘cold welding’ by which the metallic bonds that hold atoms together in each object effectively ‘bridge the gap’ between them to create a single solid object.

Electron beam welding, which requires a vacuum rather than shielding gasses to protect the weld, has potential utility when welding outside of the spacecraft, but creating an isolated vacuum within a spacecraft makes interior welding a problem.