Everything you need to know to make an invisible gelcoat repair, from preparation and filling to topcoat and polishing

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Step 1: Preparation and matching the colour

The first job in any gelcoat repair is to check the damage closely, both inside and out. Sometimes a hard knock can cause delamination within the hull, in which case a more serious repair will need to be carried out first. In this example damage to the hull at the stem is fairly typical of a modest dockside shunt, and proved to be cosmetic rather than structural.

If the edges hadn’t been sharp we’d have first had to remove any loose material with a Skarsten scraper. As it is, in this case we can simply give the damage and the surrounding area a good wipe over with acetone to remove any wax or dirt.

Before any repair is made you need to know the true colour of the hull – there’s no point matching a repair to a faded pigment which will change when it’s polished. Ideally, then, you need to polish the hull back to as close to its original colour as possible. Chalking and superficial scratches had already been polished out of our example boat’s hull, so we have a true colour to work to. If it had been an untreated hull we’d have needed to prepare a small area first.

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1. First choose a part of the hull identical in colour to the damaged area. This will become the mixing palette on which to colour-match the gel coat. Go over the area with 1,000-grit abrasive paper to remove scratches and chalking.

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2. Using a rubbing compound, the area is then flatted to restore the original colour of the gel coat, leaving it with a slightly matt finish.

 

3.Enough plain white pigment is decanted to complete the entire repair. A small amount will be placed onto the prepared palette area.

3.Enough plain white pigment is decanted to complete the entire repair. A small amount will be placed onto the prepared palette area.

 

4.The hull may appear white, but look closely to see what colour tints might be necessary for a perfect match – in this case a hint of green, with a trace of yellow and a touch of cream.

4.The hull may appear white, but look closely to see what colour tints might be necessary for a perfect match – in this case a hint of green, with a trace of yellow and a touch of cream.

 

5. Using a mixing stick, place a few dabs of uncatalysed white gel coat onto the hull. These will become your testers (because they haven’t been activated with catalyst, they can later be wiped off with acetone)

5. Using a mixing stick, place a few dabs of uncatalysed white gel coat onto the hull. These will become your testers (because they haven’t been activated with catalyst, they can later be wiped off with acetone)

 

6. Now for the tricky bit. A tiny dab of the dominant colour is placed on the end of a cocktail stick...

6. Now for the tricky bit. A tiny dab of the dominant colour is placed on the end of a cocktail stick…

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7. … and is then mixed into the tester. Care is needed to ensure all the pigment is evenly distributed, which requires some dexterity.

 

8. When you are confident you have the green element of the match, you can then switch to the next colour, cream, and repeat the process. If the match goes wrong, start again.

8. When you are confident you have the green element of the match, you can then switch to the next colour, cream, and repeat the process. If the match goes wrong, start again.

 

9.Throughout the process, keep a mental note of how much of each pigment you have used for the match. When you’re happy with the result, use this experience to multiply the ratios to make a full batch of gel coat, enough to complete the whole repair.

9.Throughout the process, keep a mental note of how much of each pigment you have used for the match. When you’re happy with the result, use this experience to multiply the ratios to make a full batch of gel coat, enough to complete the whole repair.

Step 2: Filling the hole

Once mixed, the colour-matched batch is then split, with some being kept back for the finishing coat. The majority will be used for the initial fill, and here a filler powder is added to bulk out the gel coat so it stays put, making sure the colour remains the same. As a check, it can be dabbed back onto the hull at any stage. It’s important that the entire repair is colour-matched and not just the surface layer. That way if the hull is abraded back any time in the future, the repair will remain invisible.

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1. The repair site on the stem is masked off to protect the stainless steel strap. Note strips of cardboard used to make small cofferdams.

 

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2. The thickened mix then receives some catalyst at a ratio of 1%. It’s important not to add any more than this to hasten the cure. The heat generated by a fast cure can actually alter the colours, making them darker.

 

Gelcoat Repairs

3. The colour-matched filler is then thoroughly stirred together to make sure all the catalyst is mixed in…

 

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4. …before being applied firmly to the holes with a flat scraper. The excess is then taken off with the scraper’s leading edge until the filler is standing just slightly proud.

 

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5. Once fully cured, the area is then sanded back flat with 240-grit paper on a sanding block. Using a block is best as pressure from fingertips and sandpaper alone can sometimes produce an uneven finish.

 

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6. With the area flatted, it can then be tidied as shown here, making the edge of the repair smooth. The masking tape stays on the stainless strap to protect it from scratches.

 

 

Gelcoat repairs

7. The next job is to go over the repair with a random-orbit sander fitted with a 400-grit disc. This will match the final blend of the filler to the rest of the hull, and give a super-smooth surface for the last stages.

Step 3: Applying the topcoat

The critical stage is to apply the topcoat. This is our last batch of colour-matched gel coat, and it is unthickened. The addition of a little wax-in-styrene (2%) before the catalyst is added will slightly improve the flow, and make the curing easier.

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1. A 2% addition of wax-in-styrene is mixed into the final batch of gel coat. It is stirred well, and then the catalyst is added.

 

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2. Now gently paint your activated gel coat over the whole repair, avoiding brush marks and making sure you get a fairly wide coverage.

 

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3. Once the topcoat has dried, usually in about an hour, the masking tape can be removed. The surface is given a wipe down with acetone to remove the wax, and also any splashes that may have got under the tape. The metal is colder than the hull, so gel coat splashes on it might still be quite soft and easy to remove.

Step 4: Final polishing

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1. Using a sanding block again, a 400-grit paper gently flattens off the topcoat, not forgetting details such as the edges around the strap.

 

 

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2. Switching to a finer 800-grit paper, the repair is now machine-sanded, paying attention to the stippled edges. Everything is done in short bursts, and with great care.

 

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3. The final stage is with a 3,000-grit abrasive disc. This can bring out a perfect shine by itself – but one final stage is yet to come.

 

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4. Polishing compound is now applied with a brush. Adding a little solvent to the mix makes the polishing compound smoother and workable for longer.

 

Gelcoat repairs

5. Using a lambswool buffing head on a slow- running rotary polisher, the repair is given a final polish to really bring out the shine.

 

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6. Job done. A layer of UV protective wax can now be applied to protect the hull – and its invisible repair – from UV fading.

TOP TIPS

  • IMG_8094_cmykwebThe catalysed gel coat will set in about 30 minutes, but the cure could be accelerated by heat from the hand. If you don’t want this to happen, hold the mixing pot by the rim.
  • Once the wax and catalyst have been thoroughly mixed, tap the container a few times on the worktop and let it stand for a few seconds. This allows any air bubbles to make their
    way to the surface. Bubbles trapped during application will leave pinprick voids in the finish.
  • IMG_8097_cmykwebUse the brush to stipple around the edges of the topcoat. This will give you a seamless merging
    of the gel coat with the surrounding hull when it has been sanded back.

Coloured hulls
This repair has been to a ‘white’ hull, but exactly the same principles apply to pigmented hulls. The only major difference in that case is that you would use a clear gel coat to carry the deep colours required, rather than a white one. To bulk out a clear coat, a colloidal silica would be used instead of the filler powder we used for this job.

Thanks
Our thanks to Aaron Logan of Small Boat Services,
Thanks also to Hayling Yacht Company for the loan of their shed.

Alternatively you can read how to do a simpler colour match using a clear kit you can buy from a chandlery below