Chemicals

Snow foam is a pre-wash. It goes on a dry, dirty car through a cannon, clings, and lifts loose grit off so you touch the paint as little as possible.

Shampoo is the contact wash that follows, used with a mitt and two buckets. They do different jobs and you want both.

It dissolves iron particles that have bonded into the paint, mostly brake dust and rail dust. Shampoo cannot remove them because they are embedded rather than sitting on top.

Spray it on, and it turns purple where it reacts. That colour is the contamination coming off.

Yes, and it matters more than the coating you choose. A coating locks in whatever finish it is applied over.

Wash, decontaminate iron and tar, then coat. Skip that and you seal the contamination underneath for the life of the coating.