GRP hull repair

[mullet]

Quick request for advice: my GRP boat has sustained minor hull damage, I’m preparing to repair. Last time I did any work with fibreglass my age began with a 1 (which is a long time ago) so I want to check I’m on the right track.

The damage was from lying too hard on a prop when the boat was hauled out (pivoty long keel on a trolley, nasty crosswind - DIY job before anyone rolls eyes at a dodgy yard - lessons were learned). The hull was deflected inwards below the waterline by around 20mm - 30mm. Once the props were adjusted the “dent” popped out overall few days and the outside of the hull looks fair - but there is a short hairline crack visible from inside the hull and there’s an area of inward deflection (of the shape of the prop) visible internally of roughly 150mm by 100mm. I assume that the combination of “looks fine outside but dented internally” indicates a degree of delamination of the GRP and a void. Early 80s boat, heavy lay up, solid GRP, no core.

Current thinking is to grind back the GRP internally with an angle grinder and flapper disk at the damaged area, and a fair space around it. Grind the damaged area until I find the void, then increasingly shallow further away from it. I’ll leave the outside alone (except for rubbing off antifouling so I can see daylight if I’m about to grind too deep from inside).

Repair with multiple layers of chopped strand mat and polyester resin, to build back up to the original thickness. Finally flow coat over the top.

Sounds relatively sensible? Any advice would be much appreciated. Age and value of the boat mean that it probably isn’t worth getting surveyors/insurers involved; and this the sort of job I’d like to be able to do myself rather than take it to a yard.

 

[chris-s]

From the description it sounds reasonable although I would use epoxy and some heavy weight mat and skip the flow coat.

 

[William_H]

I presume there is a bulge on the inside. Yes grind this down and then a bit more. As said use epoxy resin with glass cloth.(not mat) Polyester will hopefully stick but not as well as epoxy. You might consider using carbon fibre for the repair material being much stronger and stiffer and might mean you do not have to grind away so much. In the end I suspect do nothing would be ok but you must be satisfied your self. So do what you have to do. Flow coat is fine to finish although you could just use pigment in epoxy. (being inside) ol'will

 

[ChromeDome]

Age and value of the boat mean that it probably isn’t worth getting surveyors/insurers involved; and this the sort of job I’d like to be able to do myself rather than take it to a yard.

Clarification of that point would #1 to check

 

[ducked]

Using epoxy for repair would be conventional, but I'm not sure I would, partly because its more expensive, but mostly because its pretty toxic and can get you seriously sensitised.

Of course in theory you can completely protect yourself, but with sticky stuff in confined spaces, in my hands, best defence is no be there.

Seems the greater strength of the repair may be an expendable edge

"I have come to prefer epoxy for laminating add-on stiffeners because of the resin’s superior adhesion,
which can be tested in an impact. However, let’s at least make an effort to bring some truth to the issue of epoxy
versus polyester.

In honest-to-god, let’s-glue-it-together-then-rip-it-apart lab tests, a polyester bond averaged around 70% of the strength of the original fiberglass laminate using the same resin. Epoxy failed at a little above 80%. That makes an epoxy bond to fiberglass
about 15% stronger—not to be sneezed at but probably substantially less than you expected. The point of this is that polyester resin has been used to make successful boat repairs for half a century, and if for budget or comfort reasons, you want to use polyester, you will not be cheaping your way to a watery grave. "

Casey, (2009) This Old Boat 2nd Edn. McGraw-Hill

Unfortunately he doesnt seem to cite those tests. Tsk Tsk

 

[PCUK]

I use West epoxy on a regular basis and have never noticed any adverse effects.

 

[ducked]

I use West epoxy on a regular basis and have never noticed any adverse effects.

Nevertheless, it happens.

Prevalence of skin sensitization and dermatitis among epoxy‐exposed workers in the wind turbine industry - PMC

and at a fairly high frequency

Occupational sensitization to epoxy resins in Northeastern Italy (1996–2010) - PMC

 

[oldmanofthehills]

Nevertheless, it happens.

Prevalence of skin sensitization and dermatitis among epoxy‐exposed workers in the wind turbine industry - PMC

and at a fairly high frequency

Occupational sensitization to epoxy resins in Northeastern Italy (1996–2010) - PMC

Thats not really the exposure profile for a one off repair. Many things are dangerous from continual and excessive exposure but harmless from occasional exposure

 

[William_H]

Or use vinylester resin. Can handle colder temperatures and adheres well. ol'will

 

[ducked]

Nevertheless, it happens.

Prevalence of skin sensitization and dermatitis among epoxy‐exposed workers in the wind turbine industry - PMC

and at a fairly high frequency

Occupational sensitization to epoxy resins in Northeastern Italy (1996–2010) - PMC

Thats not really the exposure profile for a one off repair. Many things are dangerous from continual and excessive exposure but harmless from occasional exposure

But they seem appropriate as a response to a post about use on a "regular basis", while not providing specific support for the harmlessness of occaisional exposure.

Indeed the second study includes mention of "a statistically significant association between epoxy sensitization and unemployed males, possibly due to unreported exposures from hobbies, small repairs, or other undeclared activities." which suggests the possibility of harm from occasional exposure

 

[justanothersailboat]

Given the price of epoxy I'd be surprised if unemployed males could afford it for hobby purposes... more likely to be exposure in some work context in an industry that subsequently declined in the study area.

In practice many people build whole boats with epoxy and don't end up sensitised so I suspect that with good ventilation and gloves and neat working the odds of a problem from the occasional repair are rather low. And polyester can be pretty noxious stuff too. But I wouldn't want to be without both, for different jobs. This does sound like potentially a job for poly.

 

[PaulRainbow]

Quick request for advice: my GRP boat has sustained minor hull damage, I’m preparing to repair. Last time I did any work with fibreglass my age began with a 1 (which is a long time ago) so I want to check I’m on the right track.

The damage was from lying too hard on a prop when the boat was hauled out (pivoty long keel on a trolley, nasty crosswind - DIY job before anyone rolls eyes at a dodgy yard - lessons were learned). The hull was deflected inwards below the waterline by around 20mm - 30mm. Once the props were adjusted the “dent” popped out overall few days and the outside of the hull looks fair - but there is a short hairline crack visible from inside the hull and there’s an area of inward deflection (of the shape of the prop) visible internally of roughly 150mm by 100mm. I assume that the combination of “looks fine outside but dented internally” indicates a degree of delamination of the GRP and a void. Early 80s boat, heavy lay up, solid GRP, no core.

Current thinking is to grind back the GRP internally with an angle grinder and flapper disk at the damaged area, and a fair space around it. Grind the damaged area until I find the void, then increasingly shallow further away from it. I’ll leave the outside alone (except for rubbing off antifouling so I can see daylight if I’m about to grind too deep from inside).

Repair with multiple layers of chopped strand mat and polyester resin, to build back up to the original thickness. Finally flow coat over the top.

Sounds relatively sensible? Any advice would be much appreciated. Age and value of the boat mean that it probably isn’t worth getting surveyors/insurers involved; and this the sort of job I’d like to be able to do myself rather than take it to a yard.

Sounds fine. Leave as big a taper around the damage as possible. Chopped strand and polyester resin will be fine, treat yourself to a roller Buy paddle Rollers from the UKs favourite supplier - Fast delivery

ECF is as good a place as any for the materials you'll need, some good "how to's" on their site too.

 

[ducked]

Seems the greater strength of the repair may be an expendable edge

"I have come to prefer epoxy for laminating add-on stiffeners because of the resin’s superior adhesion,
which can be tested in an impact. However, let’s at least make an effort to bring some truth to the issue of epoxy
versus polyester.

In honest-to-god, let’s-glue-it-together-then-rip-it-apart lab tests, a polyester bond averaged around 70% of the strength of the original fiberglass laminate using the same resin. Epoxy failed at a little above 80%. That makes an epoxy bond to fiberglass
about 15% stronger—not to be sneezed at but probably substantially less than you expected. The point of this is that polyester resin has been used to make successful boat repairs for half a century, and if for budget or comfort reasons, you want to use polyester, you will not be cheaping your way to a watery grave. "

Casey, (2009) This Old Boat 2nd Edn. McGraw-Hill

Unfortunately he doesnt seem to cite those tests. Tsk Tsk

Might be these tests by West, which report similar numbers.

WEST SYSTEM Epoxy vs. Polyester

 

[Stemar]

While there may be individuals who are highly sensitive to epoxy, wearing gloves and long sleeves will be enough to protect most people, and anyone with any sense will do that whatever resin they're using - they're all horribly sticky and hard to remove from anything.

OP's description of the damage suggests that he could probably ignore it and be fine, unless he were unlucky enough to get a hard bump in the same place, so it should be repaired. The difference in price wouldn't be huge, so I'd go with epoxy because it's what I'm familiar with, but the relatively small difference in strength mentioned above says it really doesn't matter.

A bit of Fred Drift - Is the difference in bond strength to wood as small? If so, why do people like Ran Sailing use epoxy when they're going through gallons of the stuff?

 

[ducked]

While there may be individuals who are highly sensitive to epoxy, wearing gloves and long sleeves will be enough to protect most people, and anyone with any sense will do that whatever resin they're using - they're all horribly sticky and hard to remove from anything.

OP's description of the damage suggests that he could probably ignore it and be fine, unless he were unlucky enough to get a hard bump in the same place, so it should be repaired. The difference in price wouldn't be huge, so I'd go with epoxy because it's what I'm familiar with, but the relatively small difference in strength mentioned above says it really doesn't matter.

A bit of Fred Drift - Is the difference in bond strength to wood as small? If so, why do people like Ran Sailing use epoxy when they're going through gallons of the stuff?

Believe the difference in bonding to plywood is supposed to be greater, though there are apparently special primers (, e.g. Primax) that improve polyester resin bond to plywood, though not up to epoxy level.

I've only done repairs to holed plywood dinghies, with polyester resin and without using any special primers, and got away with it