My chassis has been fully galvanised too. I've painted the outer surface and Im planning on filling the internal, hollow box sections with high density expansion foam. The foam is simply to prevent the boxsections filling up with water, mud and debris. i.e, as soon as you go wading, the chassis will fill with water, muddy water usually, and that mud will remain and collect in the chassis for years to come.
The expansion foam I have is a high grade, high density cell structure so will not absorb water. Similar to that used on the APGP One Ton amphibian.
The foam bonds well with the chassis so any slight flexing of the chassis will not cause foam to break-up. Box section, mild steel chassis arn't designed to flex anyway.