what would be a good international standard to refer to
I claim to be no expert in international (or even American) standards, but a quick search on my company's database gives me some options that may be worth considering.
The front part is exposed to the elements so it will need to be painted to protect it from corrosion.
I assume you mean that the housing will be steel since you are concerned about corrosion. Consider having a look at the following standards:
ISO 12944-5, Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Part 5: Protective paint systems - Second Edition
ASTM D1014, Standard Practice for Conducting Exterior Exposure Tests of Paints and Coatings on Metal Substrates
ASTM MNL20, Corrosion Tests and Standards: Application and Interpretation - 2nd Edition
specifying the corrosion resistance of the paint and the final products?
You mention paint, but a galvanized coating may be fitting for your application as well.
ISO 1461, Hot dip galvanized coatings on fabricated iron and steel articles - Specifications and test methods - Third Edition
ASTM A123, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products