Q&A – Fibreglassing method 3 – Method for large areas

Fiberglassing larger areas is done differently from small.

Large areas require that the glass be draped dry over the surface then the epoxy is applied over the glass and worked through the surface.

The example here is glassing a centreboard but the principle is the same for much bigger areas like the bottoms, sides and decks of bigger vessels.

The exception for the dry method application of the glass is with some heavy “stitched” cloths like biaxial and triaxial.  If hand laying up these thick heavy cloths it can be hard to get the epoxy right through the cloth.

Sometimes with moderate sized areas it can make things go smoothly to precoat the surface – but if the precoated epoxy goes off or you are not organised in laying out the fiberglass smoothly a gigantic mess can result.

If in doubt use this dry application method.

When happy with the shape and smoothness it is time to glass – pay particular attention when making to decision to glass that the first 30% of the leading edge should be pretty smooth.  When you run your hands around the leading edge there should be no bumps or hard edges – your good work here will result in much better boat performance later.  Fix up any problems using the torture board or a long block as described before.

Generally I do foil glassing in two steps allowing the epoxy to cure after the first before doing the second:
1.Do the tip – see method for double bias taping
2.Do the body

It is possible (but slightly more risky) to do the tip first immediately followed by the body

If wanting to do it in one step I predrill a hole for a long gyprock (drywall screw in the tip of the foils before they are glassed.

Then when the tip is glassed as below the screw can be put in and the foil suspended between two sawhorses as in the section “glassing setup for the foils” below.

Glassing the tip
(See the section on use of double bias tape)

Glassing the tip of the centreboard or rudder using double bias tape.

Glassing the tip of the centreboard or rudder using double bias tape.

When you are epoxying you only apply resin to the areas where you want the cloth to stay – with these foils the epoxy should only go about 20mm (¾”) past the tip.

Stand the foil with the tip upwards and clamp to a sawhorse or in a vice so it can’t fall over.

First brush epoxy on the tip of the foil plus about 3/4” and then lay the bias tape on the surface.  Use a brush with a gentle dabbing motion to apply epoxy until the glass goes clear in the defined area.

Each time the resin goes tacky add another coat until the weave of the cloth is filled.

Allow the epoxy to cure then using 180 sandpaper and a cork block sand the end smooth and matte ready for the flatter surfaces of the foils to be glassed.

Draping glass
So talking standard small boat weights of woven cloth here – no fancy weave or stitched cloths – and weight under 15oz (500gsm).

The glass is laid over the area usually with the assistance of gravity.   Pull the cloth around so it covers the area you want.  If it threatens to slide off the surface I use a few pieces of masking tape to hold in position.

If you want to rough trim the glass to shape you can at this point, but be aware that the glass might move a bit while you are working it so I would leave a good margin or plan to trim off excess later when the epoxying is around 80% done.

One thing to be aware of is the selvage edge.  If you look at the manufactured edges of cloth off the roll you will see two or three fine threads that look different from the other threads.  These are to help hold the cloth together.  They don’t go clear when epoxyied and will also leave a line in the surface when sanded.  So find the end and pull them out of the cloth sideways – they have done their job.  it is not necessary to remove them if they are going to be trimmed off later.

Glassing the body
I usually set the foil up with the leading edge facing upward by running long drywall screws at least 25mm (1”) perpendicularly into each end of the foil.  The screws need to be about 50mm (2”) behind the leading edge.

The screws can then be placed between two sawhorses with the foil suspended between them.  The gap between the sawhorses and foil needs to be no more than 12mm (1/2”).  Place a piece of scrap timber over the heads of the screws and clamp it to the sawhorse (or screw it down with a couple of screws.

Epoxy the first layer of glass

Using a squeegee to apply the first coat of epoxy to the fiberglass.

Using a squeegee to apply the first coat of epoxy to the fiberglass.

Mix epoxy and roll onto the glass.

Starting from the centre of the leading edge roll the glass into even contact with the blank.  Working from the middle out to either end and from the front to the back pulls the glass out and stops wrinkles from forming.

As you get to any pieces of masking tape you used to hold the glass in place just remove them.

Glass goes clear when just enough epoxy is added.

Glass goes clear when just enough epoxy is added.

The glass will go clear when the epoxy is rolled or squeegeed  in properly.  Add only enough epoxy to do this – the texture of the cloth weave should still be visible.

Fill the weave
Wait for the epoxy to go sticky  – if more epoxy is added before the first coat has become sticky the glass will float up on top of the epoxy and the foil won’t finish smooth..  Roll a coat over the glassed surface.

If the roller is held so it cannot turn and lightly pulled across the epoxy it will smooth the surface – as you do this the cloth has to be kept tensioned by working from the middle out to the ends and from the front to the back..

When it goes sticky roll on another coat, skid the roller to smooth the surface.  Continue adding coats as the previous one goes tacky until the weave is filled.  Runs may occur if you don’t allow the previous coat to go tacky enough.  With 200gsm (6oz) cloth usually there are a total of three to four coats of epoxy required to fill the weave.

Trimming the glass when the epoxy is cured
When the epoxy is cured (usually overnight) the excess glass can be trimmed off.  Work carefully around the foil trimming down excess to a few millimetres.

The final trimming will be done with sandpaper and a cork block.

Keep hands well away from glass edges.  If you drag your skin along a raw glass edge it might cause a cut.

Final Finishing

When cured the epoxy is sanded smooth try not to cut into the glass much.

When cured the epoxy is sanded smooth try not to cut into the glass much.

Some people say you can’t get a good finish with epoxy.

While the epoxy should be painted or varnished to protect it from UV light and it is possible to get better finishes than epoxy with those surfaces the quality of the finish can still be good.

In fact, getting a good finish in the epoxy saves labour when you move to doing the paint or varnish coats.

Some say you cant get a smooth finish using epoxy

Some say you can't get a smooth finish using epoxy

There are two pics right.  The first is the foil with a sanded epoxy finish lying to the left of an unsanded foil, the second is of a final epoxy finish before a lighter resanding and painting or varnishing.

The reason for the final coat of epoxy is to seal any glass fabric that is exposed during the sanding.

Leave a Reply