Building requirements outlined by NHBC and LABC, only advocate the use of austenitic stainless steel or PVCu beads in all external rendering applications, anywhere in the UK. Galvanised steel beads are no longer accepted for external rendering in any situation.
Although it’s generally known for its corrosion-resistant properties, galvanised steel can react with the chemical components in render, which will lead to quality issues, from unsightly rust stains, to the degradation of the bead and possible cracking in the render.
This is why we only recommend the use of stainless steel or PVCu products for external applications. Galvanised steel beads should not be used externally.
According to NHBC
NHBC 2021: CHAPTER 6.11 - Render - Section 6.11.7 Detailing, Ancillery Items
"Stop beads and render stops should be austenitic stainless steel or PVC."
According LABC
LABC 2020: Chapter 6 External Walls = Section 6.2.7 Materials
"For bellcasts, other beads and stops; PVCu bead or stainless steel bead is acceptable. For other beds and stops, a PVCu bead or stainless steel bead is acceptable."