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By Bruce Niederer — GBI Technical Advisor
Above: The layout of an Optimist pram provides an example of how to go about estimating epoxy amounts.
This formula will help you estimate the amount of mixed epoxy needed to wet out fiberglass cloth (assuming a resin-to-fiber ratio of 50:50) and apply three rolled epoxy coats to fill the weave of the cloth, i.e. “fill coats.”
The formula includes a waste factor of approximately 15%; however, more (or less) may be needed depending on the job and personal application technique. The epoxy is applied at standard room temperature, approximately 72° F.
Gallons of mixed epoxy=A× [(Wf×0.00085)+0.0075]
Where:
A=Total area covered by fiberglass. Units are in square feet (ft2)
Wf =Total weight (W) per square yard of fiberglass (f) cloth used in laminate. Units are in ounces per square yard (oz/yd2), i.e. 6 oz fiberglass cloth weighs 6 oz/yd2.
Let’s use the Optimist prams in my previous article as an example:
| Bow: 322 in2 | Transom : 507 in2 | Side×2:3784 in2 | Bottom:3444 in2 | Total sq in: 8057 in2 |
| ÷ 144 | Total outside sq ft: | 56 ft2 | ||
The inside is covered with 6 oz fabric and three fill coats. The bow box is covered with 6 oz fabric on one side only.
| Bow: 322 in2 | Bow box: 720 in2 | Transom:507 in2 | Side×2:3784 in2 | Bottom:3444 in2 |
| Total sq in: 8777 in2 | ÷ 144 | Total inside sq ft: | 61 ft2 | |
Outside calculation 56 ft2 [(12 oz/yd2 × 0.00085) + 0.0075] = 0.99 gal
Inside calculation61 ft2 [(6 oz/yd2 × 0.00085) + 0.0075] = 0.77 gal
Total 1.76 gal mixed epoxy
Note: a Group Size B 105 Epoxy resin and hardener makes 1.2 or 1.3 gal of mixed epoxy depending on hardener.
