Fibreglass boat repairs.

DIY FEATURE - Invisible mending


Step-by-step guide: how to repair fibreglass boat gelcoat

1. Select a matching colour from the paint shop.


2. Make the final colour selection.


3. Identify the gelcoat cracks that need repairs.


4. Identify the holes that need filling too.


5. Dig out the old holes and cracks with a sharp knife.


6. Try the refined lumpy fibreglass gelcoat repair brew.


7. Mix plenty of tint from the paint shop into white putty.


8. Plug the holes on the underside with tape or blue-tac.


9. Once it's semi-cured, it will be much easier to carve off.


10. Once filled, compare the colour. If it's too light in shade you will need to add more tint to get it right.


11. With more tint added, you eventually get perfect fibreglass invisible mending.


DIY fibreglass repairs

You’ve got a problem with your boat and you try different shops, hoping for simple expert advice. But they can surprise you with their answers, which are sometimes helpful but as I have found, often evasive. Expertise can be hard to find, and my problem stumped them all.

“I have a fibreglass boat,” I said to the acned youth in his V8-pitstop uniform, his skinny frame slouched over the broom. A glazed look misted over his eyes. Undaunted I continued, “and the gelcoat needs fixing. It’s got some holes and cracks to fill, so I need some coloured bog please.” Well, that really threw him.

“We sell car-things here, mate,” he declared, looking around at oils, spanners and plugs. “Not boats. You gotta go to a boat shop for that sorta thing.” He had dispensed his little bit of wisdom, and looked relieved. He turned to drive his broom further along the aisle, lining up with the next chicane of merchandise.

I wasn’t finished yet and held out a can of filler.

“Says here,” I pointed, “that this gelcoat repair putty is for fibreglass, and that colour pastes may be added.”

“Where exactly,” I asked politely, “do I find that colour paste?”

He scratched his chin, looking for inspiration in acne. “We don’t sell colour here mate. No-one asks for it either, so try a boat shop.”

I questioned him further and increased his obvious discomfort. But NO, he didn’t know what colour the putty was. And NO, he couldn’t open the tin — against company policy.

So I looked closely at the fine-print on the label. It said the white putty could be tinted and would change to pastel shades. I turned to show Young Helpful, but he had motored around to the next aisle. The putty was expensive for such a little bit of polyester, but I felt the need to give it a try and all I wanted now was colour.


Gelcoat repair

I went to a hardware-chain where experts are always glad to help. At the Fillers and Sealants aisle I found a can of polyester filler and the instructions also said that colour can be added. This product, I thought, could be worth trying too, being happy to experiment a bit.

I collared a cruising assistant and popped the colour question. “Sorry mate, we haven't got colours for fillers,” was the reply. But for boats? “Well… we don’t do boats,” he explained smiling apologetically, trying to be helpful, “You could try the paint-counter, though. They might be able to help. Aisle Six,” he said, pointing.

I found the paint-counter. Hundreds of colours looked like rainbows on display. I had a good eye for colour so pocketed five swatches that, from memory, looked close. You need to be careful with colours. They can look different under fluorescent or incandescent lights. It’s an exacting process, so I took them home to my trailersailer for matching. They were close but not quite right, so I kept the two closest colours and tried the paint aisle again.

This time Gold Chateau C13.BB13 looked good; the numbers I knew had meaning… to a paint mixer. But, to be absolutely certain, I checked it against the boat again — perfect in the sun and in the shade. Adding pigment is irreversible so it’s best to get it right.

Then back at the paint-counter I asked for colour tints, for polyester gelcoat or resin. “Dunno mate. This is a paint-counter, we don’t do boats. Have you tried a boat shop?”

I insisted, pointing to the can. “I bought this today and it says here, that colour can be added!” I explained.

“Yeah, but we only sell paint, so I can’t guarantee it’ll work,” replied the shop assistant.

“Never mind the guarantee,” I said, frustration rising, “can you pour the colours into a little pot? I’ll mix it in myself.”

Self-conscious, I took the tints in a little jar that looked vaguely like a urine sample. Back home I coloured little dobs of putty and filler, and they both changed to grey; nothing like the yellow-ochre that I fondly call nappy-yellow. No guarantees, but was he right? I had spent $60, and for my troubles the experts had said to “go to a boat shop”. Problem was I had seen the colour range before, in boating catalogues, where you find bright blues, reds and yellows.


What do you?

Surf-shops were suggested, too, but when I phoned they couldn’t help either. Finally, a shipwright remembered a guy who worked on plastic boats. These experts are apparently hard to find, and that’s strangely comforting. It meant that fibreglass needs very little maintenance!

I rang him at once and he explained, “You can’t get that sort of thing, only premixed colours, but they never match. So I fix the gelcoat and spray the boat in two-pack. Looks good as new. I’ll give you a quote if you like.”

Suspecting the worst, I asked for a ball-park figure instead, describing the 16-footer as a daysailer with a little cabin. The estimate was astronomical; half the value of my little boat. And then he said I’d only need to paint it every 10 years! With each new setback my gelcoat was looking better and better, and it had mellowed very nicely with age.


Flowcoat vs gelcoat

I had had more than enough advice about boat shops, so still doubtful I visited a ship’s chandler, just to make certain. He listened and reluctantly pulled a dog-eared colour-chart from under the counter, like a well-thumbed pornographic magazine. He would trust me with his only copy and if I selected a colour, he would get it in. But he advised me to buy flow-coat instead. I'm always learning!

Flow-coat is similar to gelcoat, he said, but with different wax properties — so it doesn’t stay sticky. The colour chart had a yellow-ochre patch that could possibly work. But half the colour chart, including the ochre, had been crossed through with felt-pen. “They don’t make those anymore,” he shrugged, “they were for old boats made in the seventies.” I felt like pointing out that old boats probably need it most, but I needed help and not an argument.

I sort another chandler for a second opinion. He recognised the old colour, sympathised, and said his own fibreglass trailersailer had gelcoat problems, too, and he couldn’t match its colours. I checked his colour chart: Inca Gold, and not crossed out. The colour wasn’t perfect, but could he check please, for availability? Anything, I said, would be better than those unsightly cracks.

We discussed the gelcoat problem and I concluded it was a manufacturing fault that had happened years ago; probably a bad batch in the mould and too much heat on setting. I was becoming an expert, too. He phoned someone in Sydney. “Don’t make that colour anymore. It must be an old boat.” I felt defeated.


Fibreglass filler for boats

Then a cheerful, grey-haired assistant butted in. Said he had some Inca Gold at home, in the shed. It was okay, too, he said, as he’d passed it through a stocking. That should have told me something! He’d find a litre for me… be here tomorrow! “Too easy, mate,” he said brightly and walked off.

I called in every day for three weeks for my rare Inca Gold. So it must have been a big shed! Back home I opened the can and found a lumpy brew inside. I stirred and stirred and it only got worse. The stuff at the bottom came up so hard and dry it must have been decades old. I was still determined to fix the cosmetics on my classic little Explorer, it’s what I do for fun and enjoyment, and my search had now taken six months, so I continued with it anyway.

My wife sympathised and offered an old fork to break up the lumps. So I prepared a doubtful batch with little lumps and poked the porridge into some test-cracks. Surprise, surprise, it actually hardened, lumps and all! EUREKA!

I scraped more cracks and holes with a sharp pocket-knife. I filled and then pared away when the putty was half-cured, then, when hard, I sanded with three different grits: 340, 800, and 2000. The colour was a lousy match up close, but stand back far enough and it looked great; the gaps were gone! I went to bed that night, reasonably happy. But my nagging little leprechaun at 3am whispered to me, “The colour still isn’t right, John. You can do better, to be sure.”

Back to square one. I wondered about the first batch of coloured putty, the grey one. Had the expert got it wrong?

So it was back to the paint-counter and another pot of tints to match my precious colour swatch — cheap at $6. The shop assistant typed the code and now I checked the screen: yellow-ochre, brown, a little squirt of green, and a massive squirt of white. “Just the colours please,” I asked. “No white this time. I reckon that should do it.”

Home again, I mixed another sample on my wife’s plastic pastry sheet. The smooth side is ideal, but get domestic agreement first. I stirred the tints into white putty until it looked just right. I realised I had come full circle, and the experts had got it wrong.

I counted 30 screw holes and dozens of cracks, but again started with a small batch. Great! The colour was right, but the shade too light; should I continue? Then the Leprechaun whispered, “Why not go all the way, John?” So I threw caution to the wind, and added still more tint, but how much can putty take? Three times that of normal paint?

I filled more holes and waited anxiously for it to set. I pared and sanded, and the colour was excellent. I was overjoyed when the holes and cracks totally disappeared, and the surface matched the mellow sheen. The result was perfect fibreglass repairs on my boat.


From Trade-a-Boat Issue 423, Feb 2012.