Why Pin Holes Spoil an Otherwise Perfect Finish

You can spend hours laying up a panel, fairing it, then priming and flatting until your arms ache — and still watch the gloss fall apart in the final coat. The culprit is almost always a surface defect so small you can barely see it before painting. Pin holes, pits, sanding scratches and tiny air voids don't look like much on a bare panel, but a glossy topcoat acts like a magnifying glass. Each one shows up as a crater, a dull spot, or a spot of "orange peel" that no amount of polishing will remove.

The good news is that these defects are easy to fix — provided you treat them before the topcoat goes on, and provided you use the right filler for the job. Filling is a skill worth practising properly, because everything downstream depends on it. A flat, void-free base is the foundation of a good finish; no paint system can rescue a bad one.

Know Exactly What You Are Filling

Before you reach for a tub of filler, identify the defect. Different problems need different approaches, and lumping them together is how people end up with filler sinking into the surface weeks later.

• Pin holes — tiny voids, usually under a millimetre across, caused by trapped air or solvent escaping from the substrate as it cures. Common in gelcoated and vacuum-bagged parts, and around tight radii where the fabric didn't wet out fully.
• Porosity — clusters of pin holes that read as a dull, chalky patch. Often the result of a poorly mixed resin, a resin-starved laminate, or a filler coat that was sanded too aggressively.
• Pits and deep sanding scratches — mechanical damage from coarse paper or a stray bit of grit under the block. These hide beautifully until the gloss highlights every one of them.
• Gouges, blowholes and dry patches — anything more than about a millimetre deep. These are structural, and they need a resin-based repair rather than a surface stopper.

Run a clean cloth over the panel, then look at it under raking light from a low angle. Wetting the surface with a little solvent or water will also make pin holes "pop" visually. Mark the offenders with a pencil so you don't lose track of them.

Choosing Between Glazing Putty and Thickened Resin

For fine defects, a two-part polyester glazing putty (often called stopper or spot putty) is the practical choice. It mixes in seconds, sands beautifully, and is designed precisely for pin holes, pinhole clusters and shallow scratches. Buy the two-part variety, not the single-part cellulose stopper in a tube — the latter shrinks, never fully cures under a topcoat, and will ghost through your gloss within a season.

For anything deeper than about a millimetre, or anywhere the repair needs to carry load, mix your own filler instead. Add a thixotropic agent such as fumed silica to epoxy or polyester resin to prevent sagging, plus a lightweight filler like glass microballoons to make it easier to sand. Aim for a consistency like smooth peanut butter: it should hold a peak on the mixing stick but still spread without tearing.

• Glazing putty: pin holes, porosity, sanding scratches, shallow sink marks.
• Thickened resin: gouges, blowholes, edges, voids over 1mm deep, repairs near fixings or load paths.
• Never: use single-part stopper under a two-pack topcoat, or bridge a deep void with a thin skim of putty.

Preparation Is Most of the Job

Filler only bonds to a surface it can key into, and it only hides defects if the surrounding area is already flat. Sand the repair area with P240 to give a mechanical key, then remove all dust with a tack cloth and a wipe of panel degreaser. If the panel is gelcoated, cut back to a dull, matte finish — glazing putty will not stick reliably to a shiny surface.

Feather the edges of the surrounding paint or gelcoat so the repair blends into a flat plane. A hard ridge around the repair will show through the topcoat no matter how carefully you sand.

Applying Filler Without Making More Work

Mix small batches — a golf ball of putty is plenty for a panel. Work it into the pin holes with a firm, pressing action using a flexible spreader, dragging across the defect at 90 degrees to the direction of the hole, then again at 45 degrees. The aim is to push material in, not to lay it on thick.

Leave the filler slightly proud of the surface. Trying to apply it flush never works; you will simply sand through the thin edges and reopen the holes. Once cured — check the data sheet, but glazing putty is typically ready in 20 to 30 minutes — you can move straight to sanding. For thickened resin, allow a full cure and consider post-curing if the part will see heat.

Block Sanding to a Genuinely Flat Base

This is where the finish is won or lost. Use a firm sanding block, never your fingers — finger pressure creates hollows that follow the shape of your hand. Start with P240 dry for shaping, then step through P320 and P400. For glazing putty, wet flatting with P600 to P800 gives a superb base.

Sand in a diagonal cross-hatch pattern, changing direction regularly, and keep the block flat to the surface. Dust off frequently and inspect under raking light. A light guide coat of primer or a dusting of dark aerosol will reveal every low spot and remaining pin hole — if the guide coat survives sanding, so will the defect. Re-fill, re-sand, and repeat until the panel is uniformly flat.

Finish by wiping down with panel degreaser and inspecting one last time. Only then should you reach for the primer. Get this right and the topcoat will do the rest of the work for you.