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Old 20-01-2010, 11:14   #1
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Deck Crazing Remedy

I am fixing up my Cape Dory Typhoon and have some gel coat crazing on the deck.

not sure which route to take. I have

1) route out cracks, fill with epoxy, then sand and paint with Interlux Brightsides single part poly. This is probably not as durable as the 2-part linear poly, but might hold up better because it is more flexible and wont crack. Maybe this isnt an issue if I fill the cracks with epoxy, but I am thinking I might only be able to fill the bigger cracks and leave the tinyest ones.

2) do the same and apply Interlux Perfection linear Poly. This would offer better abrasion resistance. A drawback is that it cannot be sanded and buffed out if you get any runs or sags (roll and tip application method)

Also I am considering sanitred for the nonskid. Is it worth the extra cost over the paint additive?
Could the whole deck be painted with sanitred and look good? I would only put the nonskid in the same areas that have nonskid now.

Tough choices...
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Old 20-01-2010, 11:37   #2
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Is it cracking, or crazing? If it is crazing, then most likely you will need to pain the surface. We did this recently, the cabin sides were crazed. We prepped, primed and painted with Brightsides. Perfection would be a more exacting, but more durable option.

Unless the cracks are very big/deep, you should not need to "Fill them". That is what the primer coats do.

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Old 20-01-2010, 11:44   #3
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Mostly just spider web type crazing. A few are bigger and you can feel them with your finger nail.
I can see primer covering them, but not filling them. They are just hairline cracks, not really open voids, unless of course you were to jump up and down next to them I am sure they would open and close with the flexing of the deck.

If I dont open them up and fil them with epoxy wont they just crak whatever primer and paint I put on top of them?

Paint is pretty rigid.
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Old 20-01-2010, 11:58   #4
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Crazing of this type usually happens because the gelcoat is actually too thick. So it expands at a different rate than the substrate. It is not because of flexing. The primer for Perfection and Brightsides will fill things up really well. The paint also fills to a small extent.

You can always test out a section before doing the whole boat.

We did ours almost 2 years ago with brightsides, and there is no evidence of the crazing returning.

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Old 20-01-2010, 22:52   #5
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which primer did you use?
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Old 21-01-2010, 08:41   #6
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Tim, we are in the process of doing the same thing with our boat.

The gelcoat cracks, as noted above, primarily because it does not expand and contract at the same rate as the fiberglass. We are grinding ours out with a dremel tool, and filling them with MarineTex. We will then paint with Perfection (our non-skid was 2 part, so we can go back with 2 part).

The biggest issue with 2 part is that you HAVE to mix and then use all that you mix. It's also nearly 2x as expensive. But, it will last quite a bit longer.

A trick we've found on the cracks is to cover them with clear packing tape. Then you can see the cracks under them. Grind through the tape, and then you have the tape in place for filling. DO remove the tape after filling. If you have an area with a lot of cracking coming together (like an impact point), you can leave some of the tape in place right around the area. It will sand off. But you don't want to sand tape most of the time! This has worked WAY better than the "tape around" technique we were using before.
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Old 21-01-2010, 09:14   #7
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Tim,
I am working on the same issues. I think the cracking is a combination of shrink and being brittle, either way it is stress being relieved in the gel coat. I have found that none of the cracks go deeper in the substrate on my boat, which is a relief.

Just make sure you do not have hard points that don’t distribute the stress over a big enough area as that will come back to haunt you with epoxy in the cracks. You could cover it over better by filling the cracks with paint that is more flexible but I prefer to do a little re-engineering on the inside of the hull and eliminate the problem rather than cover it up.


I use about the same process, though I give it all a couple thin coats of West System epoxy and sand flat before priming. This being a slow process and life gets in the way, I now have some of this type of repair that is about 18 months old and shows no deterioration.

Um Saudade
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Old 21-01-2010, 09:20   #8
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The cracks are mostly at corners like the corners of the cockpit. If the problem is merely swelling and shrinking of the gel at different rates than the glass beneath then I would think the 2 part poly would be fine. Maybe just grind and fill the bigger cracks and rely on the primer for the rest.

Just when I was convinced to use the 1 part
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Old 22-01-2010, 09:21   #9
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If your deck/non skid has been previously painted, you'll want to make sure the 2 part won't lift it. Get some 2 part solvent (see if someone has some you can "borrow"), and put it on a rag, place it on your deck, and cover with plastic for 24 hours. If it shows any signs of damage, use 1 part.
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Old 23-01-2010, 05:27   #10
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Quote:
Originally Posted by Tim_H View Post
The cracks are mostly at corners like the corners of the cockpit...
"Stress Cracks" develop at corners.

Boat hulls are usually made from the outside-in, by applying the gel coat to a waxed mold and then adding the layers of glass reinforcement and polyester to complete the hull. The gel coat, the pigmented and filled polyester, is used to hide' the underlying glass composite structure, to color the hull, to produce a flexible surface which acts as a shock absorber and to help keep water from diffusing into the composite. Blisters and de-lamination are caused by water diffusing into the hull and reacting with water soluble material to form a droplet of solution which, because of osmotic pressure, grows in volume and creates a force which results in a blister or a de-lamination.

The integrity of the hull surface - ie: gelcoat - is essential in preventing this disastrous water diffusion into the laminate. The Gelcoat is basically a resin-rich surface (much like a person’s skin), which is designed to:

1. protect the laminate from the environment
2. reduce fibre pattern
3. provide a smooth aesthetic finish
4. eliminate the need for painting

Hairline cracks in a gelcoat surface (hereafter crazing) are often (mistakenly) considered merely a cosmetic problem. However, on occasion, gelcoat cracking may be an indication of underlying structural problems, or a result of manufacturing defects, environmental, or operating conditions.

Allowing cracks to remain “open” would be a serious misjudgment, likely resulting in future serious structural & cosmetic problems, including de-lamination and osmotic blistering.

Whatever the underlying causes (or source); the mechanisms causing gelcoat crazing are always STRESS and MOVEMENT.

Gelcoat, by nature of being on the outer surface of a structure, is subject to the highest strain of the entire laminate. The tensile or compressive strain in a loaded laminate increases with distance from the neutral axis of the load. Under a flexural load, the highest tensile strain is recorded at the top surface, while the highest compressive strain is at the bottom surface. There is no strain at the interior of the laminate, at the neutral axis. Because of the critical positioning of the gelcoat film in a laminate structure, both the laminate and the supporting structure must take into account the strain imposed by anticipated operating loads.

There are a number of sources of localized stress in a boat hull, all of which could first appear as crazing or hairline cracks, and may ultimately lead to structural de-lamination, and/or blister formation and growth.

1. Stresses are produced by polymer shrinkage during curing. As the laminating resin cures it bonds to the solid gel coat and then shrinks on curing producing a tensile stress in the laminate near the gel coat interface. After the gel coats are cured on a mold, the resin is applied. It bonds to the gel coat before it cures. The resin near the gel coat interface goes into tension as the resin away from the interface cures and shrinks. Undercure, resulting from under-catalization, low shop temperature or too thin a film, will usually produce a flexible gel coat. While this flexible gelcoat is not prone to cracking, it may be inclined to premature color degradation, loss of gloss, chalking or chemical attack. On the other hand, over-catalization can easily lead to a brittle gel coat which cracks with little provocation.

2. Stresses are produced by swelling of the resin due to water diffusion. The amount of water present causes swelling of the polymer. The resin can swell as much as 10 percent by volume, and this is greatly affected by the degree of cross linking. Stresses are generated by differential swelling. If the entire hull swells uniformly, no differential stress will result. However, if one layer swells and the adjacent layer does not, the adjacent layer will be pulled apart (put in tension) by the swelled layer. The level of differential stress generated will be determined by the water gradient and discontinuities in the gradient and not by the absolute amount of water present.

The stress is transient. The maximum tension will move inward and decrease in magnitude as water diffuses. If the resin has high strength, that is, it is well cured, highly cross-linked, and reinforced with glass, it can survive the passing stress field and not crack. If a disk crack forms, it constitutes a vacuum. Any local WSM units will be drawn toward the crack to increase the pressure. This is a mechanism for concentration of WSM units in the vicinity of the crack. Stress cracks can create blister centers.

3. Stresses are produced during boat use. Peak stress is produced by wave action, rigging stresses, impact stresses and buoyancy stress.

4. Internal cracks produce stress concentration sites at the crack tips which can lead to further cracking or accelerated chemical attack. Strictly speaking, the crack does not produce a new stress but intensifies one of the above three stresses. Cracks can magnify a stress by hundreds of times.

5. Thermal shock or direct sunlight can heat darker colored composites to beyond the heat distortion temperature of the resin causing warpage, creeping of built in stresses, over expansion of trapped air or moisture - causing laminate separation (de-lamination), blistering, or even catastrophic collapse of entire structure.

Two or more of the above five types of stress can interact at a particular point in time and space. For example, if a modest shrinkage stress combines with a small water swelling stress and at the same time, severe wave impact flexes the hull, localized disk cracking can take place. Furthermore, the reaction of the polyester resin to the stresses applied is dependent on the flexibility and toughness, i.e. resistance to cracking, of the resin. If the resin is brittle cracking will occur. A flexible resin can deform under peak stress loads without cracking. Resin flexibility depends on the type and number of links in the polyester chain and, very importantly, on the number of cross-links between the chains.

To reiterate: the mechanisms causing gelcoat crazing are always STRESS and MOVEMENT. Movement in one form or another can have a number of causes. Many times the cause of the movement can be determined from the pattern of cracking.

There are a number of types of cracks that are evidenced in gelcoat, and each type may signify a particular problem or set of problems. Various crack configurations may indicate the underlying causes, and are vital in troubleshooting the problem. In some cases the root problem has nothing to do with the gelcoat, and is a manifestation of a structural problem or unanticipated movement of the substrate.

Radial Cracks:
Usually associated with impact, radial cracks are a good indicator of the direction of the impact. The classic "spider" crack is a result of a reverse impact or sharp, localized stress riser. a frontal impact is indicated by a concentric circle pattern, with the diameter of the inner circle having a relationship to the size of the impacting object.

Linear Cracks:
There are two groups of linear cracks: stress field patterns and parallel stress cracks. The primary cause of these cracks is flexural strain. However, in the case of stress field cracking, either structural elements or local stress risers modify the parallel pattern into a more complex structure.

Parallel stress cracks indicate flexural movement perpendicular to the direction of the cracks. Parallel curvillinear cracks often indicate a distribution of stress over a supported panel surface. If the surface is restrained in two 90-degree planes, the flexural strain will "fan out," creating a "palm leaf" effect.

Parallel stress cracks radiate from a localized nucleation. The main effect is the deflection of the laminate inward toward the restraining member. The parallel stress crack is interrupted by a stress concentration around a point

Convergent stress field cracks may result when flexural strain is interrupted by a structural member.
Divergent stress fields occur when the laminate is deflected away from the supporting member and the crack propagation is consolidated through a localized lack of movement.

Thermal fatigue Cracks:
Thermal fatigue cracks are a result of repetitious expansion and contraction of the gelcoat film. Whether in a parallel pattern or an isotropic (nondirectional) configuration, thermally induced cracks are characterized by short discontinuous sections, and are usually grouped in forming in a dominate stress field.

Isotropic thermal cracks are a result of the surface expanding and exerting a tensile strain within the gelcoat film in a unidirectional fashion.

Parallel thermal fatigue cracks usually are propagated by expansion of the surface in conjunction with localized flexural stress.

Form stress risers:
This type of crack is a result of an intervening shape, usually a cutout, in the surface of a panel. The form or shape serves to concentrate strain into a localized area.

In the case of a hard point riser, a low-level strain may result in cracking due to high-level stress concentration in a very small area. A square shape with sharp corners is a prime candidate for creation of a hard point riser.

A radial riser may have a different origin. In this case, often a bolt or hardware fitting exerts a tensile force in the area around a hole. The edge of the hole distends causing a tensile failure of the gelcoat in the surrounding area.

Repairing Gelcoat Crazing and Hairline Stress-Cracks

Some TEMPORARY quick Stop-Gap Measures [pun intended ]

First: Roughen the damaged area and clean it with acetone. As with all sealing, painting (and the like), PREPARATION is everything.

“Captain Tolley's Creeping Crack Cure” is a one part water based acrylic polymer low viscosity penetrating sealant that uses capillary action to find its way inside a fine crack and sets by water loss forming a rubbery mass. It usually takes 24 hours to cure (air drying). It is recommended to apply a fill every 20 minutes till no product is absorbed into the crack. Because it has poor bonding strength, and can only accommodate a small amount of movement of the structure, I would only use it as an immediate TEMPORARY measure.

“Cyanoacrylates” ( like “Krazy Glue”) are acrylic resins (C5H5NO2) that cure almost instantly. The only trigger it requires is the hydroxyl ions in water. Super Glues may help to stop a crack from spreading, but will not fill nor seal a crack. Drill a small (about twice the diameter of the crack’s width) at each end of the crack, then dribble in some Super Glue. Again, only a TEMPORARY stop-gap.

See also Jeff’s excellent tutorial:

A Primer on Fiberglass Construction
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Old 23-01-2010, 07:18   #11
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You can also "tie" everything together by sanding all of it down then re-glass. Lotta work but it will fix the problem.
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Old 26-01-2010, 09:41   #12
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A trick we've found on the cracks is to cover them with clear packing tape. Then you can see the cracks under them. Grind through the tape, and then you have the tape in place for filling. DO remove the tape after filling. If you have an area with a lot of cracking coming together (like an impact point), you can leave some of the tape in place right around the area. It will sand off. But you don't want to sand tape most of the time! This has worked WAY better than the "tape around" technique we were using before.
Great Tip!

Thanks,
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Old 26-01-2010, 13:22   #13
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Something we found this weekend - don't leave the tape on more than a few hours: tape, grind and fill and remove the same day. Also, on flat areas, just place the tape down - it becomes very hard to remove if you press it down too much.

I was previously doing the taping, and the Admiral wanted to do that part. She's a school teacher. Nice and smooth and adhered VERY tightly!
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Old 26-01-2010, 13:48   #14
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FGCI.com or store has a cool trowel on epoxy fairing compound. Rolling a tight bead in front of a trowel can give a surface that won't need a whole bunch of sanding. I'd acetone scrub fair out, 100 grit block sand, 220 grit surface condition prime 220 scuff paint. I did my nonskid with gelcoat. I 80 grit sanded the old to remove paint, taped out nonskid areas, acetone scrub, then mix your colored and roll a thick coat on and just keep rolling with a good quality heavy knap roller till it doesn't want to roll anymore. It forms a rough texture that looks great with a little knocking down.
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