How to Select the Right Primer for Steel Aluminium and Galvanised Substrates

Applying topcoat directly to bare metal or porous substrates without proper primer selection leads to adhesion failure and premature coating breakdown.

1. Determine When Priming is Required

Prime whenever bare metal (ferrous or non-ferrous) is exposed. Prime when the substrate is porous or alkaline, including fresh plaster and new concrete. Prime when the product TDS specifies a primer system. Previously painted surfaces in sound condition may require spot priming only over exposed bare areas rather than full system re-priming.

2. Select Primer Based on Substrate Type

For mild steel, structural steel, and cast iron, use FPW15 Red Oxide Primer. This primer is formulated for ferrous metals only and must not be applied to aluminium or galvanised surfaces. For aluminium and non-ferrous metals, use EPS15 Etch Primer, which provides chemical etch adhesion through the aluminium oxide layer. For galvanised iron and IBR sheeting, use GI Primer, which is specifically formulated for zinc-coated surfaces and suitable for both new and weathered galvanised steel. For concrete and masonry substrates, select an alkali-resistant formulation required for new concrete and plaster.

3. Apply Primer According to Specifications

Apply primer at the wet film thickness specified on the product TDS. Ensure even coverage across the entire surface, paying particular attention to edges, corners, and welds on metal substrates where coverage gaps commonly occur. Allow the primer to cure fully before applying topcoat, following the touch-dry and recoat windows specified in the TDS.

4. Manage Recoat Timing for Optimal Adhesion

Respect recoat windows to maintain inter-coat adhesion. Applying topcoat too soon while primer has not fully cured compromises adhesion. Applying topcoat too late after primer surface has degraded also compromises inter-coat adhesion. Reference the TDS for specific timing requirements for each primer system.

Key Insights

Primer selection must match the substrate chemistry - ferrous metals require different primers than aluminium or galvanised surfaces. Proper wet film thickness application and adherence to recoat windows are critical for long-term coating performance. Previously painted surfaces in good condition need only spot priming over bare areas, avoiding unnecessary full re-priming that increases cost and application time.