Don't get a thick oil, tar or wax-based rustproofing. Moisture gets in between the rustproofing and the metal, causing more rust. Also, those products are too thick to flow into narrow gaps.
Instead, get an annual spray of a thin liquid by Krown rustproofing or Rust Check.
Also, swap the engine before you paint the fenders. No matter how careful you are trying to keep them covered, you'll end out with some scratches.