What exactly is GEL COAT

[Petrolia]

I've been seaching for information about what 'gel coat' actually is. There are lots of posts about repair & cleaning but I don't really even know what it is !

I understand it is the outer layer and therefore it goes in the mould first but that's about all.

Is it just GRP without the glass mats ? How thick is it ?
If my hull is blue is it blue right through or is the blue just a thin skin ?

I read about cutting the gel coat back. I assume this means using grinding paste of some kind to take a layer off ? Can this be done on blue hull as well as white ? How much comes off ?

It seems that 'gel coat' is soft and easy damaged. Why don't manfacturers make the 'skin' hard ?

All & any education appreciated.
Petrolia

 

[byron]

From Wikipedia, the free encyclopedia


A gelcoat is a material used to provide a high quality finish on the visible surface of a fibre-reinforced composite material. The most common gelcoats are based on epoxy or unsaturated polyester resin chemistry. Gelcoats are modified resins which are applied to moulds in the liquid state. They are cured to form crosslinked polymers and are subsequently backed up with composite polymer matrices, often mixtures of polyester resin and glass fibre or epoxy resin with glass and/or carbon fibres.

The manufactured component, when sufficiently cured and removed from the mould presents the gelcoated surface. This is usually pigmented to provide a coloured, glossy surface which improves the aesthetic appearance of the article.

Many marine craft, including large yachts, are manufactured using composite materials and the outer layer, or gelcoat, is typically 0.5mm - 0.8mm in thickness. Gelcoats are designed to be durable, providing for example, resistance to ultraviolet degradation and hydrolysis.

Specialised gelcoats can be used to manufacture the moulds which in turn are used to manufacture components. These require very high levels of durability to overcome the mechanical and thermal stresses encountered during the curing and demoulding processes.

Suitable resin chemistries for the manufacture of gelcoats vary, but the most commonly encountered are unsaturated polyesters or epoxies. Within each of these categories, the resin chemistries are further subdivided.

 

[boatbuilder]

Your blue boat should only be blue in the gel coat. lay up resid should allways be clear so that you can see any dry spots or air bubbles.
Never buy a boat that has been layed up with pigmented resin.
Indeed some yards use clear gelcoat below the waterline as well, for the same reasons.

 

[leftofcentre2006]

To expand on this, is it just the Gel Coat thats the waterproof bit?

I seem to remember reading this somewhere. I.e. if it gets chipped, water can seep between the laminates yes?

Regards

Andy

 

[Dave_Snelson]

All of Byron's stuff is technically correct. There are some clues in there as to why gelcoat is applied. Making your boat look nice is the main one. GRP itself is waterproof and does not require gelcoat to complete this process - check out a RN Minesweeper, it is made of GRP for obvious reasons, but has no gelcoat coz it doesn't have to look nice. Indeed, gelcoat is the cause of osmosis - which RN minesweepers do not suffer from.

 

[Petrolia]

Thanks Dave, Boatbuilder, Byron,
So the 'gel coat' is literally a non matted coating to give a good finish.

So I can 'cut back' or polish the blue but I've only got about 0.5mm or 20 thou before I'm thru the 'gel coat'.

So is the superstructure built the same way with a 'gel coat' or only the hull ?


Petrolia

 

[Dave_Snelson]

All visible surfaces are gelcoat.

 

[tcm]

yep, as DS says, hull and lots of superstructure are grp with gelcoat. They have moulds into which they put a release agent, then gelcoat, then layer upon layer of glassfibre-and-resin depending on how strong the part in question needs to be.

You can get gelcoat neat, of course, and it can be sprayed.

If you look inside hull, under sofas or whatever, you'll see "rough" grp. Same thing with a grp bath - the inner surface is gelcoat.

 

[leftofcentre2006]

So then what is Osmosis?

 

[tcm]

it's another thread, i think?

 

[Gludy]

"GRP itself is waterproof and does not require gelcoat to complete this process "

A surveyor told me today that it is the gelcoat that is the waterprrof bit - the GRP acts as a sponge to water ....... just what i was told today .... so who is right?

 

[markdj]

I always knew that the gelcoat is the waterproof part but water will soak it after a year which is why it's good to have a GRP boat out of the water for a couple of months to dry out the hull. Alternatively, you can use Gelshield which will protect the gelcoat more from water ingress.