The quality of a paint job on a fibreglass part is almost entirely determined by the preparation work done before paint goes on. Fibreglass parts arrive raw or in gelcoat — they need cleaning, sanding, priming, and blocking before any colour coat. This guide walks through each stage so you get a clean, professional result.
INSPECT THE PART FIRST
Before any prep work, inspect the part in good lighting. Look for pinholes in the gelcoat, surface texture, high or low spots, and any damage from transit. Mark problem areas with a pencil or chalk. These are the areas that will need the most attention during priming and blocking.
CLEAN THE SURFACE
Wipe the entire surface with wax and grease remover (e.g., Sika Cleaner 226 or equivalent). This removes release agent residue from the mould — if not removed, primer won't bond properly. Don't skip this step.
SCUFF SAND WITH 180 GRIT
Using 180 grit sandpaper (dry or wet), scuff the entire surface to give the primer a mechanical key. You're not trying to remove material — just scratch the surface. Sand until the shiny gelcoat surface is uniformly dull. Wipe clean with a tack cloth.
FILL PINHOLES
Apply flexible spot filler or fibreglass-compatible polyester filler to any pinholes or low spots. Let it fully cure, then sand back flush with 120 grit, finishing with 180 grit. Repeat as needed until the surface is smooth.
APPLY HIGH-BUILD PRIMER
Apply three coats of 2K high-build primer, allowing each coat to flash off before applying the next. High-build primer fills minor surface texture and scratches. Apply in even, overlapping passes from about 25–30cm distance.
BLOCK SAND THE PRIMER
Once the primer is fully cured (usually 2–4 hours at 20°C), block sand using a flat sanding block with 320–400 grit wet-and-dry paper. Keep the block flat and sand in straight lines to reveal high and low spots. Any low spots will appear as shiny areas; fill and re-prime as needed. This stage is the most labour-intensive but is what separates a show-quality finish from an average one.
FINAL PREP BEFORE COLOUR
Sand the entire primed surface with 600 grit wet-and-dry, wipe with a tack cloth, and the part is ready for colour coat. Ensure there are no sand-throughs (bare fibreglass showing through the primer) — spot prime any bare areas and re-sand before proceeding.
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