F80 M3 Underside Protection

All vehicle's eventually corrode - fact!

The first area to start seeing corrosion on a vehicle is the underside.
This is because the underside gets far more than just rain, it gets:
Rain, Trapped Mud & Road Grime (causing dampness), Salt, and regular/daily Condensation during temperature fluctuations (usually early morning), not to mention being hit with Stones.

Luckily, all modern BMW's (since 1st Jan 2004) are covered with a 12-year anti corrosion warranty. 

However, "The Warranty does not cover corrosion caused by neglect, accident damage, stone chips or other external influences".

The key words being "stone chips or other external influences".

Because neither you nor BMW can prove what's going on under your car whilst you're driving nobody can prove you haven't suffered stone chips to the underside and/or driven over unfavourable ground and introduced minor defects to your vehicle.

Why do stone chips make a difference?
Well, the underside of your car's chassis (at the rear) is NOT coated with traditional Seam Sealer.

BMW's assessment:

Visually inspect the underbody, wheel arches and frame members for rust and damage.

If damage to the underbody is identified:

Rectify identified faults after consulting with customer and issue a separate invoice.