Fibreglassing

Based on my own experience I thought I would share a few thoughts on DIY fibreglassing of koi ponds.
There are numerous pitfalls for the unwary and the purpose of this letter is to highlight some of these.

Firstly, what is often done is to just buy and use the polyester resin obtainable from the builder's merchant or from the motor spares dealer. This is absolutely fine for making a box gutter which will be waterproof. However, during curing some volatile components escape from the surface exposed to the atmosphere and curing doesn't occur in this very thin surface layer. You will notice that this surface always stays slightly tacky to the touch (until a layer of fine dust etc sticks to it.) If you now fill a pond where this is all you have used, water is absorbed into this layer and it goes milky. Quite possibly, toxic components from this layer can also leach into the water.

This problem is overcome by using the standard polyester resin for the glassing (it has been optimised to have excellent wetting and flowing properties). But you must then overcoat it with so-called Flowcoat or Poolcoat, the application done very soon after the normal resin has started curing to ensure a good bond. If you wait more than a few hours you have to sandpaper the surface before application. The Flowcoat is not the same as gelcoat which is applied as the first layer in a mould. Flowcoat has had a "wax" added to stop the surface layer curing problems noted above. In addition it may require a different concentration of MEKP catalyst to cure (2%). It is also much thicker and can usually be purchased with the colour pigment already added. It leaves a smooth, hard, non sticky surface to which water can safely be added. However, if one layer of the Flowcoat is inadequate (and it usually is), you have to apply the second one before the first one has fully cured. In warm weather we are often only talking half an hour or less here. Otherwise it is tedious sanding all over again.

The roller or brush that you are using cannot be held over to the second application because it will gel and harden together with the layer you have applied. So it has to be washed in acetone, soap etc. before you can mix and apply the next layer. If you mix more than you can comfortably apply in 20 minutes it starts to gel and becomes unusable while you are still applying it.

If your method of application is less than perfect the layer may later de-laminate from the pond surface. Great skill is required with a rigid, ribbed, aluminium roller to ensure that the glass matt is mated perfectly to the rendered surface without leaving bubbles anywhere. Leaks are not infrequent and funny things like osmosis into trapped air bubbles occurs.

The health aspects of working with resins with highly volatile components are not inconsiderable. You really need to wear a respirator and gloves the whole time.

Working with “chopped strand mat”, the form of fiberglass cloth usually employed, has several problems of its own. While wetting it out with resin the short- length fibres often come away and seem to get everywhere except where you want them. After the resin has hardened, needle sharp points seem to protrude everywhere. Whenever I work with glass fibres I itch for days.