Estimating Epoxy Amounts

Estimating Epoxy Amounts

By Bruce Niederer — GBI Technical Advisor

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]

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 pram in the previous article as an example:

The mast step was modified to create an airtight box in the bow which adds buoyancy in case of a capsize.

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.