Gel coat & its problems (Part II)
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On this page we will go into much greater detail on why we use gel coat,
what it's made of & what causes its problems
Why do we have problems with gel finishes?
Without using a gel coat glass fibre products would be little more than transparent mouldings, with the weave just below the surface. This is not just a cosmetic issue, the epoxy resins which make up the structure of our gliders are susceptible to attack by UV from the Sun. With a transparent moulding the UV would be able to penetrate much further & damage would not be limited to a thin layer at the surface. While making bio-degradable gliders might appeal to the eco-lobby it is unlikely to become a major selling point from the manufacturers. The structure therefore needs to be protected from sunlight.
The gel also allows the structural fibres to be buried some way below the surface. This reduces the effects of shrinkage which occurs over time with epoxy resins. It also gives us a boundary layer in which any cracks which start to form can be removed before reaching the underlying structure. This layer also allows us to use fine wet & dry paper to achieve the required level of finish without risk to the fibres underneath.
In summary gel coat allows for the product to take on a colour, achieve a high gloss finish and protects the structure from the effects of UV.
So what is this magic material? Originally it was ordinary resin coloured & thickened with fillers. Modern gel coats are highly complex products that protect the structure and enhance its finish.
Current gel coats are more than just resin. They contain pigments - White gel uses Titanium Dioxide - ground to a powder and suspended in a suitable medium, to give them colour . UV reducers are added to stop sunlight from penetrating through the gel finish and attacking the structure underneath. Fillers and thixotropic agents are added to stop sagging when applied to vertical surfaces. Dilutants are used to reduce the viscosity for ease of application. Other additives for plastisization, degassing and reduction of fish eyeing are also included.
The white stuff on your glider is
therefore a highly complex materials capable of protecting it for
many years.
However they are still based on polyester
resins.
Why not epoxy ?
Gliders are built using epoxy resins as these are stronger and
more resiliant than the polyesters used in our gel coats.
So why don't we use epoxy resins for our gel coats?
One perceived advantage is that we could do away with the gel
coat altogether and use the same product for both structure &
surface finish. Unfortunately all those nice things we added to
our gel to allow it to do its job also compromise its properties
as a structural resin, so we'd still need a separate structure
& gel coat.
As mentioned above, one of the major roles of gel coat is to
protect the underlying structure from the effects of UV. Epoxy is
more prone to attack by UV, so a much greater proportion of UV
blocker would have to be added. This then compromises the
properties that we want in a surface finish.
Finally polyesters now have such a head start over epoxies in the development of gel coats that it is unlikey that anyone will put in the time, money & research in to allow them to catch up
As gel finishes age they are subject to embrittlement and resultant cracking. This is due to photo-initiation which occurs during exposure to the UV in natural daylight. The affected surface is subject to a chemical change, the surface hardness increases and at the same time the inter molecular bonding degrades. This same process also causes a change in colour.
The microscopic contour of the surface plays a role in the resistance of the gel coat to these effects. A smooth surface, in microscopic terms, will have much less area of ridges and valleys than a rough one. Presenting a smooth surface dramatically reduces the effects of UV degradation as there is less area exposed.
Polishing the surface will fill the valleys and will harden to a new smoother topography at the approximate level of the average peak.
The reactivity of the surface determines
how it will interact with other materials in the environment.
Observing how water lays on a surface is a simple indication of
surface reactivity. A droplet on a
Under the microscope, polyester gel coats appear to be very porous materials. Each pore creates a relatively sharp transition with the surface, this sharp edge is subjected to increased photo-initiation. These holes are also collectors of micro-debris resulting in increased chemical reaction & degradation. Obviously keeping this to a minimum is essential.
Briefly then our gel is subject to attack
from UV in sunlight & chemicals in the environment.
To minimise the effects we need to keep the surface smooth at the
microscopic level, to reduce surface area, and to reduce surface
reactivity.
The easiest way to do this is regular hard waxing.
For more information on how to look after your gel, what to do if it is starting to crack, or on refinishing your glider go back to Gel Finishes Part I