Blisters and pinholes in gelcoat

Pinholes, blisters and pits are not the same thing

The words get used loosely, so it helps to separate them. Pinholes are tiny round holes, often no larger than a pinprick, that appear in gelcoat from air trapped during the original application. They show as dark dots, especially after the surface has been waxed or washed. Pits are small craters, often irregular, formed when the surface has been etched by chemicals, abrasion or long exposure.

Blisters are raised domes under the surface. They can be as small as a pencil eraser or as large as a coin. They form when moisture enters the gelcoat or the laminate and creates pressure beneath the surface. Some are filled with clear liquid and some with a cloudy or colored fluid that can smell sour or like vinegar. Blisters are most common below the waterline, but can appear on any surface that stays wet.

Gelcoat can also show crazing, a network of fine cracks, and star cracks around impact points. Crazing is a different issue, and it is covered with other surface damage. The point of this page is the three that look like small defects in an otherwise smooth surface: pinholes, pits and blisters.

Why they form

Pinholes usually trace back to manufacturing. When gelcoat is sprayed into a mold, tiny air bubbles can be trapped, or the film may not wet out fully. After the part is demolded, the bubbles open and leave pinholes. They tend to appear in clusters on flat panels, such as the cockpit sole or a transom, and they get filled with dirt, making them easy to see.

Pits can develop with age. Long-term exposure to ultraviolet light, salt and acidic cleaners wears the surface and leaves tiny craters. Hard water spots left in the sun can etch the finish. Harsh cleaners, especially acid-based products used without rinsing, eat into gelcoat and leave a roughened or pitted texture.

Blisters are a moisture story. Water vapor passes slowly through gelcoat and the laminate beneath it. Where there are water-soluble materials in the resin or cavities in the laminate, osmosis draws water in and builds pressure. The blister is the result. Boats kept in the water for long periods, especially in warm water, are more prone, and so are those with an older or thinly applied barrier coat.

How to tell them apart on your own boat

Start by looking at the pattern and the location. A cluster of tiny dark dots in a flat area above the waterline is typical of pinholes. Scattered rough spots in a high-exposure area suggest pitting. Raised domes, especially below the waterline or on a surface that is often wet, point to blisters.

Touch is useful. Run a fingertip over the area. Pinholes feel like tiny dimples or nothing at all. Pits feel rough. Blisters feel like bumps. Press gently on a raised bump. A blister may feel slightly soft or may sound hollow when tapped lightly with a plastic tool. Never puncture a blister, because it can release liquid and open a path for more moisture.

Photographs help a lot. Take close pictures with side light, so the texture shows, and one wider picture to show the pattern. Include something for scale. Send them to a detailer or a repair shop when you call, and note whether the marks are above or below the waterline.

• Tiny dark dots in clusters on flat panels: pinholes from application.
• Rough, crater-like spots in sun-exposed areas: pitting from wear or etching.
• Raised domes, often below the waterline: blisters from moisture.
• Fine network of cracks: crazing, a separate issue.
• Sour or vinegar smell from a bump: a sign of fluid within a blister.

What detailing can honestly do

A detailer works on the surface of the gelcoat. Cleaning removes dirt from pinholes and pits so they look less obvious. Compounding and polishing can level shallow pits and smooth a rough surface, and a sealant or coating then protects it. For a boat with very light pitting from etching, a polish can bring back a large part of the lost gloss.

For pinholes, the answer is more limited. A pinhole is a hole in the gelcoat, and polishing does not close it. Compounding may reduce the edges and make the dots less visible, and a coating can fill the faintest ones slightly, but the holes remain. A detailer will tell you so, and the right fix is a gelcoat repair shop that fills the holes with matching material and refinishes the surface.

For blisters, a detailer cannot repair them. A blister is a structural and moisture issue. Cleaning around it is possible, and a detailer can keep the area clean while you decide what to do. Polishing over a blister can break it open or hide an early warning sign. If you see raised bubbles, stop and consult a professional.

When to involve a fiberglass repair shop

Any blister deserves a professional look, especially if there are several, if they are on the hull below the waterline, or if they are growing. A repair shop or a marine surveyor can test the area with a moisture meter, determine whether the blisters are in the gelcoat or deeper, and recommend a plan. That plan might involve opening, drying, filling and sealing the blisters, or in more widespread cases, stripping and applying a barrier coat.

Pinholes in large numbers also benefit from a shop. Filling them with gelcoat or a filler product and then sanding and polishing produces a smoother, longer-lasting surface than any polish alone. A shop that matches gelcoat color can blend the repair so that it does not show.

Cracks, star patterns and chips are different damage and also belong to a repair shop. A detailer can clean and polish around a repair after it has cured, but should not attempt structural work. We say so plainly when the line is crossed, because honest advice protects your boat and your budget.

Blisters and the bottom of the boat

Hull blisters are mostly a bottom concern. They are found beneath antifouling paint, in the barrier coat or the gelcoat that was covered by paint. The yard sees them when the boat is hauled, and it is often the yard or a surveyor who first reports them. They are not usually something a detailer would see unless the hull is bare or the bottom paint is thin.

If you were told that your boat has blisters, ask for details: how many, how large, whether they were opened, and whether moisture readings were taken. The right response depends on those facts. Some owners monitor small blisters over time. Others choose a full treatment. That decision belongs to the owner, the yard and the surveyor.

What a detailer contributes is the finish above the waterline, which should be kept in good condition. A well-maintained topside also helps a surveyor or buyer see the boat clearly. Read our page on post haul-out detailing for how the haul-out and the detailing visit fit together.

Pits from cleaners and the lesson they teach

Some pitting is self-inflicted. Strong acid-based cleaners, sometimes sold for removing waterline stains or rust, can etch gelcoat if left too long or used too strong. The result is a dull, rough patch where the cleaner sat. Owners who try a product on a stubborn stain sometimes discover that the stain is gone but so is the gloss.

Abrasive cleaners and scouring pads do similar damage, and so do dry wipes of dusty surfaces. The finish becomes micro-scratched and eventually shows as a haze. Compounding can fix this if the damage is shallow. A detailer will test the surface and tell you how much material can be removed safely, because gelcoat is a finite layer.

The lesson is to start gentle and never leave a chemical on gelcoat longer than the label says. If a product is not marked safe for fiberglass, do not use it. Our page on why household cleaners damage boats covers the common mistakes.

Protecting gelcoat that already has pinholes or pits

A surface with pinholes holds dirt, so it benefits from regular washing and a good protective layer. A sealant or coating forms a barrier that keeps dirt from settling deep in the holes and makes it easier to rinse out. Without protection, the dots fill with grime and look worse each season.

A coating can reduce visible pinholes slightly by filling the faintest ones, but it does not repair them. Our ceramic coating page explains what a coating does and does not do. A detailer should never promise that a coating will hide defects. It improves gloss and protection, nothing more.

If you plan to sell the boat or prepare it for a survey, consider repair first, then detail. A buyer or surveyor who sees filled and refinished gelcoat will take a more favorable view than one who sees a field of pinholes under a fresh polish. A shop's work is the foundation, and the detail is the finish on top.

Two boats, two different answers

Picture a white center console, several years old, with a cockpit sole that shows dozens of tiny dark dots after every wash. The dots are pinholes from the original gelcoat application, and the boat has never had a moisture issue. The owner wants a cleaner look before putting the boat on the market. The honest plan is a thorough clean to pull dirt out of the holes, a fill by a gelcoat shop for the worst panels, then compounding, polishing and a sealant. A detailer alone could make the dots less obvious but not remove them.

Now picture a cruiser kept in the water year round that comes out of the slip with a scatter of raised bumps along the lower hull. Some feel soft and one weeps a cloudy liquid with a sour smell. This is a case for the yard and a surveyor, not a polish. The owner should stop any cosmetic work on the bottom, ask for moisture readings and decide on a treatment with the yard. The detailer's part comes later, on the topsides and above the waterline, once the hull work is settled.

These two examples show why the same word, bubbles or holes, can lead to opposite advice. Looking closely, photographing the marks and describing them in plain terms is the fastest way to get the right trade involved.

Planning the work in the right order

If you have a mix of problems, such as pinholes, light pitting, oxidation and dirt, the order is clean, repair, correct, protect. Cleaning shows what is really there. A repair shop handles holes, blisters, cracks and chips. After the repair has cured, a detailer compounds and polishes to blend the finish, and a protective layer goes on last.

Doing it out of order wastes effort. Polishing and waxing before repair means the repair shop has to remove the protection. Repair after detailing risks undoing the polish. Telling both trades what the other will do helps them plan, and helps you schedule.

Related pages include compound and polish, gelcoat repair versus detailing and scratches on gelcoat. They give more detail on where detailing ends and repair begins.

What to have ready when you call

Describe what you see: the size of the marks, whether they are raised, flat or pitted, where on the boat they are and how long you have noticed them. Say whether they are above or below the waterline. Photographs with side lighting and a coin for scale are very helpful. If a survey or a haul-out report mentioned blisters or moisture readings, share what it said.

Quotes are given only by phone. A larger job, or one that needs a close look at the surface, may need an on-site visit. A service call charge applies for that visit and is explained on the phone before anyone is dispatched. We will tell you what polishing can improve and when a fiberglass shop should look first.

Call to describe the marks and set up the next step.

Questions people ask

Related pages

What a technician looks at under raking light

Pinholes and blisters can hide in plain sight until the light hits them from the side. A technician often uses a handheld light held low and close to the surface, which throws small shadows from every imperfection. In this view, a pinhole shows as a tiny dark point, a blister as a raised dome with a soft edge, and a pit as a shallow crater. Looking this way lets us map how many there are, where they cluster, and whether they follow a pattern, such as running along a seam or sitting on one side of a hull.

Touch comes next. Gloved fingers run lightly over the area to feel whether a bump is firm, soft, or hollow. A very gentle press tells us whether the spot flexes. We never poke or drill, because any hole opened into a blister can let moisture into the laminate and make the situation worse. If a blister looks active, seems to weep fluid, or covers a wide area, we stop and recommend a fiberglass repair professional. Our role is to describe honestly what we see and to avoid pretending a cosmetic detail can solve a structural question.

With the map and the touch test done, we discuss options in plain terms. Light pinholes in a topside can be cleaned out and protected, so dirt does not pack into them and show as dots. Larger problems belong to a repair shop. We can also advise on the order of work, such as sending the boat to a repair shop first and detailing after, so the finish is not damaged by sanding dust. The scope is described before work begins, and any quote is shared first.