In the late 1960’s NASA developed zinc silicate coatings to protect the largest mobile platforms:  CT-1 and CT-2 Spacecraft Transport Vehicles. These massive vehicles are still going 50 years later.

The GREY paint is NASA Zinc Silicate: HRZS

Corrosion protection: In order for those craft to last all of these years with a single coat of zinc silicate. They survived countless 6000 degree launches, endless days of standing next to the oceans, tons of acid, thousands of pounds of chlorides, and little to no maintenance.

November 21, 2003: Mobile Launcher Platform (MLP) number 3 and a set of twin solid rocket boosters, atop the crawler-transporter, inch along the crawlerway in support of the second engineering analysis vibration test on the crawler and MLP.

https://www.gizmodo.com.au/2015/03/massive-spacecraft-transport-vehicles-are-still-going-after-50-years/