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How much epoxy does it take to fill fiberglass cloth

I was running out of epoxy resin and wanted to know if I needed to get more or if I had enough. This is what I found out.

Are you fiberglassing bare wood?

If resin and cloth is being applied to raw wood or plywood, or for that matter to other porous substances such as some foams, some of the resin will be absorbed by the wood and will not be available to saturate the cloth.

Fiberglass comes in many forms and weights.

Once the wood has absorbed resin then whatever you put on is available to wet out and cover the glass cloth.

Fiberglass cloth comes in many weights. From the lightweight 2 ounces used more as a sanding layer or as a layer to minimize print-through of rougher cloth below it to the heavyweight 36 oz biaxial cloths of commercial manufacturing.

The weight of the cloth is measured in ounces per square yard that is 9 square feet of cloth. or in the metric world cloth is measured in grams per square metre.

Glass and other composites come in several different types of weave. The most commonly used in everyday amateur boatbuildig is plain weave. Plain weave cloth has the same number of strands in each direction.

General Formula for coverage of Epoxy resin to fiberglass cloth.

A general formula is to plan a ratio of 1:2.5 resin. This means for every ounce of cloth, you use 2.5 ounces of resin in a total of about 3 coats.

A more accurate range would be 2 or 3, to 1 depending on how you apply the resin and on how thick your laminate is. Several layers of cloth will take up more resin than just one layer. On the other hand, carefully squeegeed coats of resin can fill the cloth with only 2 to 1 ratio.

It usually takes about 3 coats of resin to properly wet out and cover the cloth. At that stage the cloth will be properly embedded in the resin but there will likely still be a pattern on the surface.

There are a few ways of avoiding this pattern. One is to mix a filler with the third coat of epoxy. Another way is to put a plastic sheet over the laminate and remove it after the resin has set. This produces very fine shiny finishes.

You know the glass is properly wetted when it becomes completely translucent. Second and third coats are needed to properly cover the weave.

Applying Epoxy

People have applied epoxy many ways. On flat surfaces, mixed epoxy is poured on the carefully laid out glass cloth and spread with a splastic spreader. I cut old detergent bottles to get a flat side and use that, but you can easily buy plastic spatulas in the hardware or paint supply stores. If you are working on a vertical surface then a small foam roller works quite well. It is also possible to use paintbrushes to apply the epoxy resin particularly in corners and in areas that are not very flat or easy to access with a spatula or roller.

If you have a choice then use a spatula. It is fast, clean and is less likely to create puddles or runs. It is the best way to get just enough resin to embed the glass and fill the hollows and no more.

Epoxy is sometimes applied using a vacuum system. This is a very good way of using a minimum of epoxy and fully saturating the composite fibre while getting a smooth finish. It is not the usual way amateur boatbuilders work because it requires specialized equipment.

What about blushing

Some epoxy create a water soluble blush after hardening. This somewhat waxy coating needs to be washed or sanded off before recoating if you have waited for more than a few hours. Barely set epoxy can be recoated without removing the amine blush.

Blushing epoxy are just as strong as non blushing types. You just have to remember to remove the blush.

Each manufacturer will have instructions and will tell you if they have a blushing or non blushing resin.

Sanding fiberglassed surfaces

Once the glass has been properly coated with epoxy it is fine to sand the surface. IF you have not filled the weave enough and you sand, you risk cutting the higher threads of the weave seriously weakening your glassed surface.

More Glassing Information

This page is not intended as a fiberglassing tutorial, I've put some links at the top of the sidebar with more information. These are articles about fiberglassing. The Gougeon Brother PDF book is particularly good, as is the FiberGlast website.

In the end experience is the best teacher.

emails: Christine

If you decide to build a boat be careful. These tools can be dangerous. If you don't know how to safely handle something find out. There are lots of forums out there with people willing to help you.

This web site reflects my personal ideas and doesn't represent anyone else's point of view. I don't claim to be an expert in anything, just some little old lady muddling along.