So, what is gel coat?
Gel coat is the surface finish applied “in-mould” to an FRP production-built boat that hides all the lumpy criss-cross matt or weave textures of the fibres and gives a smooth coloured finish.
Firstly, does my boat have gelcoat or paint?
If your FRP boat is a production boat it will have gelcoat of some sort, however it may also have been painted over at some point post-moulding (after it comes out of the mould).
As mentioned, the gelcoat will have been applied to the mould surface, so you will notice that moulded lines (boot -top lines, hull band colours, stripes) will be smooth, whereas a painted stripe or band will have a join line however hard to detect (this maybe very hard to detect on a quality ski/wakeboard boat with effects and may have a clear coat painted over everything).
Some production builders also produce all moulded products in white and paint the topsides etc. to suit the customers preference. Paint systems based on urethane and acrylic technologies have had major advancements in recent years and products such as Awlgrip™ for example offer outstanding flexibility, longevity, stable and varied colour/effects range, and ultra-violet/abrasion resistance, also in the right hands an excellent off-the-gun finish.
I think my boat is custom built, does it have gelcoat or paint?
No. Umm! Yes. Well, I mean maybe!!!!!
Most custom-built FRP boats are built in a similar fashion to a timber or steel boat where a backbone and framework will be fabricated, a core material fitted onto and over the framework. A combination of fibre in a resin matrix applied to the core to attain the engineered laminate. This becomes the outer skin of the shell (hull), this laminate will generally be engineered with more laminate on the underwater surface area and will have additional boundary bondings through stem, keel, transom, sheer and chine (if the vessel has chine lines).
At this point we have a rather ugly greeny looking fibrous mess, not something you would be proud to show your mother, but it should, at this stage be nicely cured (gone hard) pretty strong and not really supported (explain later) by the framework underneath.
So, this is now where you realise you really should have taken a bit more time in making the framework perfect and been 100% with fitting the core material.
Fairing, yup the bit nobody likes.
What is needed now is to fill and sand, fill and sand, fill and sand using the longest sanding board you can get onto the job (common to have sanding boards with 3-4 people on a single board). Starting with coarse grit abrasive paper and gradually getting finer (the sanding boards are not affectionately known in the industry as TORTURE BOARDS!!! for nothing).
The filler for a product has to have good adhesion, flexible, strong, easily screadable (think I just made that word up 😊) and impervious to water ingress (waterproof). Often, to find a filler with all these qualities makes it hard to sand*.
Once the filling is completed with an appropriate filler, it is usual practice to apply a high-build paint filling system (torture board out again, with finer abrasives), followed by a primer (block or direct-action orbital sanding), an undercoat (probably more sanding) and finally the topcoat (please let it be good enough off-the-gun).
Custom-built production boats constructed of Epoxy resins (this will be apparent by the extra mortgage needed to buy it (before epoxy converts start, I know the cost of the hull is only a small percentage of the build), I will not mention the added build time) will have epoxy fillers and paint systems and may be top-coated with two pack urethanes.
Ah so its paint!!!!!
Well not necessarily, probably but not necessarily.
There are two other methods where a custom build may have gel coat;
- The builder has decided to finish the boat after the filling stage* with gelcoat. Gelcoat can be applied quite thick and can be boarded to a high-quality standard. The product is very hard to sand compared to the paint systems, and too much thinning can cause poor weathering.
- Also, it is possible to make a negative plug (temporary mould) to build a custom-built boat, although this is more advantageous for small production runs. In this instance gelcoat is applied to the negative plug surface as with a production build.
Now we have ascertained we have gelcoat, what is it?
Basically, gelcoat is resin with three main additives.
Pigment providing the colour
Fillers providing thixotropy and body
Ultraviolet stabilisers providing protection from ultraviolet rays
Oh, I see! But what is resin?
Most production gelcoats around 90% are based on polyester resin chemistry, however some are based on vinyl esters.
Polyester Resin
Unsaturated polyester resin is produced by the esterification (chemically changing a compound into an ester) process of dibasic organic acids and polyhydric alcohols.
Don’t be confused, it helps when you realise that polyester is polyester the stuff that your buttons, shirts, ropes are made of just a different processing, well kind of.
The best one that helped me to understand was Poly Vinyl Chloride (PVC) it’s the stuff used for the plastic drain pipes and guttering. The faux leather (vinyl) office chair that you sit in at work, that’s PVC too. The main difference being the processing, the plastic drainpipes are unplasticized or uPVC and the vinyl has a plasticiser in it. Look at that monomer used styrene yup polystyrene base.
Elements used in the formulation of polyester resin are most commonly:
Unsaturated Acid maleic anhydride
Saturated Acid orthopthalic, isopthalic or adipic
Glycols ethylene, diethylene, propylene, dipropylene, neopentyl.
Monomers styrene, MMA, vinyl toluene, alpha-methyl styrene
The Dibasic acids and Glycol are put through a cooking process at up to 220° C for 14-24hrs which turns them into an ester and boils out the water. The resulting polyester polymer is a solid which is in turn dissolved in a monomer (usually styrene also having an unsaturated portions) to provide a liquid form which can be poured by the fabricator. Viscosity dependent on the percentage of monomer (between 35-60% hard to get the actual figures from the manufactures, I mean if they told us that, we would all be in our backyard making resin 😊). An inhibitor is added to the mixture to prevent the unsaturated acids from reacting and an accelerator (promoter) to speed up the curing process. The chemical composition is altered by the type and concentration of these acids, glycols and monomers to provide the ultimate properties of the polyester resin.
For example;
Isopthalic acid and neopentyl glycol provides good resistance to corrosion/chemical
Neopentyl glycol and MMA producing good weathering & UV resistance
Adipic acid results in a more flexible resin.
Terapthalic acids = high chemical resistance (used in underground fuel tanks)
Dicyclopentadiene (DCPD) reduces styrene levels and has a higher HDT (Heat Distortion Temperature) and less shrinkage with fast gel-cure cycles. However, it has few active bond sites post-cure.
Outcome?
In short, if you want a top-quality gelcoat for the parts of a boat exposed to UV rays and weathering an ISO/NPG with MMA will do the job nicely.
How do I look after the Gelcoat?
Like you would look after your car (presuming you wash and wax it of course). Gelcoat will deteriorate overtime by weathering and oxidisation. If the surface is regularly protected by a wax solution it will greatly extend the life of the surface finish automotive products are very good and easy to use, Meguiars ™ have quick wax products that are easily applied to a wet or dry surface and wiped off.
When the gel deteriorates a light cut and polish with a cutting compound or gel restorer will usually bring it back like new.