Gelcoat

[lustyd]

Hi all, I've a little question about GRP On these forums people speak of gelcoat as if it's some mythical creature which cannot be replaced, is very protective of the underlying structure, is the water barrier etc.

Now the question is, am I wrong in thinking this is just the layer of resin put down which happens to be touching the mould? Flowcoat is effectively the same thing but contains wax to make up for the lack of mould? The resin around the fibre is identical apart from containing fibre?

I'm baffled, but perhaps there is something special which a forumite can explain (they usually do!).
Thanks all
Dave

 

[VicS]

Hi all, I've a little question about GRP On these forums people speak of gelcoat as if it's some mythical creature which cannot be replaced, is very protective of the underlying structure, is the water barrier etc.

Now the question is, am I wrong in thinking this is just the layer of resin put down which happens to be touching the mould? Flowcoat is effectively the same thing but contains wax to make up for the lack of mould? The resin around the fibre is identical apart from containing fibre?

I'm baffled, but perhaps there is something special which a forumite can explain (they usually do!).
Thanks all
Dave

The gelcoat resin is not the same as the resin used for the underlying laminate. It is specially formulated to give the properties you'd require for the outer layer.

A bit of Googling should find you some info.

 

[lustyd]

The gelcoat resin is not the same as the resin used for the underlying laminate. It is specially formulated to give the properties you'd require for the outer layer.

A bit of Googling should find you some info.

I've Googled, Google says it's just slightly thicker to make it stick to vertical surfaces which says to me it's basically the same stuff. I'm probably wrong, hence asking you bunch of clevers

 

[Rossynant]

Not being current with industry, but gelcoat has many additives beside: something to make it thixotropic (to stay on mold surface), pigments, fillers, may have UV filters, air release agents etc. Apart from those it may be the same resin as in laminate below or a bit more water resistant kind.
But with this "very protective" I would argue Certainly protects from sunlight and wear, and that's about it, in long run at least. It's less elastic then laminate and brittle. But thick layer would break, no to mention cost of material. So they make it too thin to give much protection. It's for looks mostly.
On my old boat there is gelcoat in old fashion: thick. 2-3 mm, or as much as happened to be slapped on, 10 mm in some places But so its cracking all over and falling off in few places, that's how I measured... But no osmosis, so maybe the protection is better after all.
There was time when gelcoats were 'strenghtened' with very thin mat, this gave (or worsened) osmosis problems.

Can be replaced. Clear epoxy or epoxy laminate layer gives better protection, as not permeable, but then epoxy is vulnerable to light. There was epoxy-tar, great for protection, widely used on steel for anti-corrosion layer; getting forbidden now. Topsides - good protection is provided by polyurethanes.

Hull may be made without. When osmosis got common with inferior resins (industry started to save on resin production, using cheapest kinds of chemicals) boats were sometimes done this way, no gelcoat, laminate then painted. But this is more costly.

 

[VicS]

I've Googled, Google says it's just slightly thicker to make it stick to vertical surfaces which says to me it's basically the same stuff. I'm probably wrong, hence asking you bunch of clevers

I am sure if that was the case there would be no need for Gelcoat resin as such. You just be able to thicken the laminating resin and use that.

I think you'll find it produces a harder surface which retains its glossy appearance for longer, is more UV resistant and impervious to water.

 

[35mm]

It's the same but with added properties. When layed up it's thicker. Although it's the first lay up in the mould, t's the final finish. So it contains levelling/smoothing agents, pigments and UV blockers. That's about it.

 

[lustyd]

Thanks guys, good info but I'm surprised at the lack of usual forum anal retentive details (no offence, you both know more than me!). This is what got me curious in the first place, very few seem to know the detail and that makes me uneasy. When we talk about electrics people go into such detail it's hard to believe they could make it up, and usually several people say the same thing. With FRP it's more like religion, everyone "knows" gelcoat is somehow better but the specifics are just not there. You've added some info but I'm surprised that it's still not that usual certainty. Even looking on CFS they have "Gelcoat" in tins but don't explain any more than that and I can only assume there must be someone about who knows the answer.
I'll give it a day or so and maybe email CFS for detail if nobody knows the specifics

 

[lustyd]

It's the same but with added properties. When layed up it's thicker. Although it's the first lay up in the mould, t's the final finish. So it contains levelling/smoothing agents, pigments and UV blockers. That's about it.

Do you know what these are? I've found normal resin pretty level and smooth even painting it on, hence the confusion. Also I've added wax and pigment to resin and got a darned good finish on a wakeskate.

 

[Rossynant]

Helped a friend to laminate his boat, where he used the same resin for gelcoat, just mixing in a pigment and bit of filler (silica maybe, not sure now) - in fact nothing more is necessary; pigment is enough for UV protection. Did flow down more then proper gelcoat from store, but not much problem other than more careful application necessary. Tixotropic agents were somehow hard to buy at the moment. Boat is about 30 years now, still in "original gelcoat".
But naturally this was proper mix, similar to what industry would make. Just 'thickening with something' may be only acceptable for filling putty. So it's not economical to make gelcoat, unless for the fun of it

This is what got me curious in the first place, very few seem to know the detail and that makes me uneasy.
I'll give it a day or so and maybe email CFS for detail if nobody knows the specifics

Got info somewhere still in books, at the attic. Or so I hope, as somebody might have borrowed. If so important - can take a look

 

[lustyd]

Helped a friend to laminate his boat, where he used the same resin for gelcoat, just mixing in a pigment and bit of filler (silica maybe, not sure now) - in fact nothing more is necessary; pigment is enough for UV protection. Did flow down more then proper gelcoat from store, but not much problem other than more careful application necessary. Tixotropic agents were somehow hard to buy at the moment. Boat is about 30 years now, still in "original gelcoat".
But naturally this was proper mix, similar to what industry would make. Just 'thickening with something' may be only acceptable for filling putty. So it's not economical to make gelcoat, unless for the fun of it

Thanks, I'm not trying to make it, and I'm certain it won't be cheaper to do so. Just after more knowlege which is half the fun of sailing, all the random topics you get to learn about

 

[VicS]

Thanks, I'm not trying to make it, and I'm certain it won't be cheaper to do so. Just after more knowlege which is half the fun of sailing, all the random topics you get to learn about

You might also like to look into the difference between orthophthalic and isophthalic resins.

The former is an economic grade used for general laminating, the latter a higher density resin with better temperature, chemical and water resistance.

At one time orthophthalic was used then boat hulls were made with isophthalic below the water line. Probably entirely isophthalic now ??

Gelcoat resin is the isophthalic type

 

[basil421]

Sad to say the resin used in gel coat manufacture has a higher water absorption than the laminating resin. This is not ideal but necessary as the gel coat, being unreinforced by glass fibre needs to me more flexible otherwise it will be prone to crack. (We've all seen star cracks I guess) For the same reason the gel coat is not applied as thickly as we would like. Fifteen thou' was the standard recommendation for gel coat back in the 70's when I was in the business.
The chemistry will have improved since then of course and I doubt anyone still uses orthophthalic resin for boat building gel coat but it will still be necessary to have a much more flexible resin than is used for laminating so the water absorption will be higher.

The water absorption is measured by casting a piece of neat resin (ie no reinforcement) about 2" square and about 1/8" thick. This is fully post cured and then immersed in deionised water for a specific period after which the weight gain is measured.

 

[William_H]

Gel coat

I am not aware of wha tis used specifically for gel coat but essentially it is resin with pigment added.
The thing about gel coat is that it is applied to the mold in a thin layer which is allowed to mostly harden.
This means that when the glass is layed onto the gel coat it can not protrude through the gel coat so all glass ends are under a layer of resin. The timing of applying the glass and resin is such that a good chemical bond is achieved but the gel coat is hard.
If fibreglass was simply laid up in the mold the bare glass can be at the surface so inviting water wicking etc.
There is nothing special about gel coat for use in a repair of an old boat. It is the material used in manufacture and works well when still able to chemically bond with substrate but is not the best material for after build repair. An epoxy has better mechanical grip than gel coat and plu urethane is even better being UV resistant. good luck olewill

 

[Rossynant]

You might also like to look into the difference between orthophthalic and isophthalic resins.

In Poland at least both are used for various gelcoats, and both resins can be obtained in thixotropic version plain, no additives. Guess would be better to use Iso for boats, but who knows? Boats were made with ortho as this was cheaper, or even by using iso for first outer layers only. No idea what is common practice now, but I'm suspicious kind. After all some boats cost more than others

BTW this company makes most basic resins for general industry, so probably a lot is sold in UK brands... just to show example, as more detailed data only in polish. http://www.zch.sarzyna.pl/eng/polyester/products_5.html

 

[Daydream believer]

I am interested in this post because last year i applied coppercoat. I had the hull sandblasted & this left 50% of antifoul in a very thin layer on the surface. The coppercoat company told me to sand this off which i did. This took a lot of the gel coat off. I queried whether i should apply a couple of coats of epoxy & the director of coppercoat told me it was not necessary as the gel coat did nothing other than form a smooth coloured surface. I applied the 4 coats of oppercoat ( needless to say the oppercoat did not work)

However, i later spoke to an owner who had applied coppercoat & he said that the firm had specifically told him to put on a couple of coats of protective gel coat first

Now i am wondering if i have b...g..red my hull up by not putting back some protective coating first

 

[William_H]

Coppercoat

I am interested in this post because last year i applied coppercoat. I had the hull sandblasted & this left 50% of antifoul in a very thin layer on the surface. The coppercoat company told me to sand this off which i did. This took a lot of the gel coat off. I queried whether i should apply a couple of coats of epoxy & the director of coppercoat told me it was not necessary as the gel coat did nothing other than form a smooth coloured surface. I applied the 4 coats of oppercoat ( needless to say the oppercoat did not work)

However, i later spoke to an owner who had applied coppercoat & he said that the firm had specifically told him to put on a couple of coats of protective gel coat first

Now i am wondering if i have b...g..red my hull up by not putting back some protective coating first

As I understand it coppercoat is mostly epoxy resin with copper particles. It should therefor function well as a protective layer for the substrate. So I would not worry about the hull protection. As to whether it works as antifouling is another question. I have not had the urge to try it.
Fouling here at the moment is fiendish. I can see weed and crustacean growth appearing between daily swims around the boat. Yes the a/f paint is now 8 weeks old and almost rubbed off and I do demand a clean bottom so it demands daily cleaning. Not a chore with heatwave conditions. olewill

 

[oldsaltoz]

I am interested in this post because last year i applied coppercoat. I had the hull sandblasted & this left 50% of antifoul in a very thin layer on the surface. The coppercoat company told me to sand this off which i did. This took a lot of the gel coat off. I queried whether i should apply a couple of coats of epoxy & the director of coppercoat told me it was not necessary as the gel coat did nothing other than form a smooth coloured surface. I applied the 4 coats of oppercoat ( needless to say the oppercoat did not work)

However, i later spoke to an owner who had applied coppercoat & he said that the firm had specifically told him to put on a couple of coats of protective gel coat first

Now i am wondering if i have b...g..red my hull up by not putting back some protective coating first

Don't worry, it sounds like your hull was treated in a manner most Coppercoat applicators work.

Do you know if they sanded the treated area after it had cured? Because the tiny copper flakes in the resin need to be exposed or it will not at all. It should only need a good pressure wash or scrub then a light sanding, just enough to expose the copper.

Because of the way some applicators actually apply this product you can end up with less copper near the new surface and only really starts to work after
a couple of sanding sessions.

If any coating is applied over the coppercoat it effectively stops it from doing it's job.

Good luck and fair winds.

 

[Daydream believer]

I did it myself
The instructions did not say anything about sanding
After 8 weeks the boat was almost unsailable due to weed
When on my trip round England I stopped at St Helier & had it lifted & washed
The bloke doing the washing( Billy Hibbs) said he had never seen a coppercoat hull where it had actully worked. I could not sand it then ( 1.5 hr liftout) so i got him to turn the pressure right up & give it 2 more hard washes to replicate some erosion of the surface. That was in June & when the boat came out in October it had some slime & a couple of bad patches, but it had been through the freshwater in the Cally Canal

When i say unsailable--- i could hardly hold the helm over due to massive prop wash when motoring & at 6 knts the stern was dragged under the water by 15 inches. Ie the boat just squatted in the water& would not go forward
Max sailing speed on a reach was 4 kts in 20 kts of wind
After the wash there was minimal weight on the helm & I could go to windward at 6-6.5 knts in 12 knts of wind
Sorry!! Off thread, but it just winds me up

 

[William_H]

Fouled bottom

I did it myself
The instructions did not say anything about sanding
After 8 weeks the boat was almost unsailable due to weed
When on my trip round England I stopped at St Helier & had it lifted & washed
The bloke doing the washing( Billy Hibbs) said he had never seen a coppercoat hull where it had actully worked. I could not sand it then ( 1.5 hr liftout) so i got him to turn the pressure right up & give it 2 more hard washes to replicate some erosion of the surface. That was in June & when the boat came out in October it had some slime & a couple of bad patches, but it had been through the freshwater in the Cally Canal

When i say unsailable--- i could hardly hold the helm over due to massive prop wash when motoring & at 6 knts the stern was dragged under the water by 15 inches. Ie the boat just squatted in the water& would not go forward
Max sailing speed on a reach was 4 kts in 20 kts of wind
After the wash there was minimal weight on the helm & I could go to windward at 6-6.5 knts in 12 knts of wind
Sorry!! Off thread, but it just winds me up

One thing for sure a boat won't go when carrying a lot of weed. But then perhaps I am obsessive.
olewill

 

[PeterWright]

Orthophthalic vs, Isophthalic

From the early days of grp boat building, orthophthalic resins were used and the gel coat differed little from the resin in the main layup, other than having pigment and a thixotropic (non - drip) agent added, the latter to stop it running down off the mould while it cured. In the early days the wax was manually applied to the mould before starting layup, but the trick of mixing wax with the gel coat was soon adopted, saving time and labour, reducing difficulty in demoulding and with no discernible impact on quality.

The first step improvement in grp building was the imposition of temperature and humidity controls in the shops where the boats were built, this came about in the1970's, although some small yards are still doing major grp repairs in shops where there is little or no attempt to control the environment (or even outside!)

In the mid 1980's, isophthalic resin, more resistant than orthophthalic to moisture penetration and more expensive, came on the scene. In response to the burgeoning problems of osmosis on some older yachts (I suspect, but can't prove, those built with poor environmental control), good builders moved to the use of isophthalic resins for both the gel coat and the first layup of glass. Some other builders, wanting to claim use of isophthalic resin without spending too much, adopted this just for gel coat below the waterline, or some other half measure. Many cheaper or home built yachts stuck to all orthophthalic as the cheapest option.

So the truth is that "gelcoat" can vary quite widely, depending on the builder.

Coppercoat is an epoxy resin loaded with as much finely divided copper particles as they can persuade the resin to hold. It's purpose is to deter marine organisms from growing on a hull. To do this, it must be constantly eroded so that fresh copper is appearing at the surface. This will not happen on a boat which lies for months in a marina with no current through it, so the boat will grow weed. Coppercoat is good for boats which are sailed a lot and don't sit still for months on end, particularly in warm weather.

Because of the copper particles in it, coppercoat offers ready paths for water penetration to its substrate, it therefore cannot act as a barrier coat against osmosis. If it did, it would not work as an antifouling.

If you want a barrier coat, the only effective system is several coats of epoxy, again applied in a temperature and humidity controlled shop. Applying it outdoors in a British climate (dew every evening, even in high summer) risks building damp into the hull.

Before even starting to apply a barrier coat, all other finishes should be removed and the gel coat matted (careful grit blasting is the simplest option, but not leaving half the old antifouling on. Epoxy on top of antifouling is a waste of money.) the hull washed at least twice with fresh water then left to dry. Any areas where moisture readings are not low will have to be gel peeled, washed again, dried out and the gelcoat replaced with epoxy (this amounts to an osmosis repair). Applying a barrier coat over damp grp just locks damp in - a waste of time and material.

Once the whole underwater part of the hull is dry, barrier coats of epoxy can be applied and the hull kept in the controlled environment until it has cured. If you're having an osmosis repair or barrier coat application done, ask the yard to provide the record of hull moisture measurements before they started and of hull temperature and air temp and humidity at 3 hourly intervals from when application started until the last coat cured. If they can't or won't, the job is substandard.

So lustyd, I'm not sure if that's anally retentive enough for you, but it's about as much as I can recall from memory after sharing a bottle of Pinot Grigio with swmbo.

Sorry fot the inevitable typos.