how much resin?

Discussion in 'Materials' started by Lew Morris, Jan 26, 2003.

  1. Lew Morris
    Joined: Jun 2001
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    Location: Pismo Beach, Ca

    Lew Morris Industrial Designer

    I'm looking for a reference for polyester resin volumes to fabric... without doing a bunch of actual, in shop, testing. This is probably an elementary question, but the answer alludes me.

    Not specifically, BUT FOR EXAMPLE:

    1 square yard of 3.0 oz mat requires ? liquid ozs. resin.
    1 sq. yard of 3.0 oz. mat + 2 sq. yard of 7.0 oz. cloth requires?
    1 sq. yard of roving requires ? oz. resign

    I know there is an ideal ratio but is it by weight or volume?

    Is it as simple as measuring the thickness of the mat/cloth combination, and calculating the volume (as the amount of resin)?
    Obviously the glass occupies volume -- but how much airspace does one allow for the resin to saturate into the reinforcement?

    Is the 'idea ratio' (usually considered 40%) a volumetric value?

    Is anyone aware of tabular information regarding resin/mat/cloth combinations?
  2. Sgray
    Joined: Oct 2002
    Posts: 16
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    Location: North Carolina

    Sgray Junior Member

    how much resin

    Hello go to there is great info on fiberglassing also here is some info off that site about calculating material

    Accurate material estimates are necessary for two reasons.First, they obviously are needed for proper ordering, material stocking, and bidding of projects. More importantly though, estimates offer the opportunity to calculate the weight or cost of the part using a variety of laminating schedules before beginning to build.

    Unlike estimating coverage when painting, resin usage will vary depending on the type of reinforcement being used. The heavier the fabric, the more resin it will take to wet it out. A good hand laminate consists of about 50% fabric and 50% resin by weight. For example, if an application requires 3 sq yds of a 4 oz/sq yd fabric (total fabric weight = 12 ounces), 12 oz of resin will also be needed. However, if 3 yards of 10 oz/sq yd fabric is chosen (total fabric weight = 30 ounces), 30 oz. of resin will be needed.

    Glass mat requires a minimum of 2 ounces of resin for each ounce of mat. Therefore, it the application calls for 20 sq feet of 1-1/2 oz/sq ft mat, it will require a minimum of 60 ounces of resin. Remember that mat is specified in ounces per square foot, where fabrics are specified in ounces per square yard. 1-1/2 oz/ sq ft chopped mat actually weighs 13.5 oz/ sq yd!

    Since there are so many possible combinations of materials, one should calculate the weight and cost of a single layer using a variety of reinforcements. These can then be added or subtracted form the theoretical laminate until the design properties are achieved.

    1) Begin by calculating the surface area of the project. Estimate irregular shapes by measuring the approximate sized rectangles necessary to contain the tapered areas. Multiply the length times the width for each rectangle, and then add all of individual rectangles together to get the total surface area of the part. If the calculation is in square feet, divide by 9 to get square yards.

    2) Make a list of each type of reinforcement being considered for the lamination. Multiply the square yards calculated above times the ounce weight of the fabric. This is the total weight of one layer of that material. It is also the amount of resin required to saturate it. When this is known for two or three different types of materials, it is possible to calculate the weight and cost of a laminate constructed from any combination of these fabrics. To convert the ounce weight to pounds, divide by 16. Those inexperienced in saturating fiberglass tend to use far too much resin. A well saturated laminate is uniformly translucent, without milky appearing dry spots, but for the sake of weight and cost, has little excess resin in it.

    3) The final step is to calculate gel coat, primer coat, and surfacing primer usage.

    All but the very lightest of molded laminations require a gel coat. This gel coat should be 15-20 mils in thickness.

    A 20 mil gel coat will require one gallon of gel coat mix for each 80 square feet of mold surface. If a lighter surface coat is desired, spray part # 1041-B Duratec Surfacing Primer into the mold in place of the gel coat.It can be applied thinner (10-12 mils), and thus lighter.Duratec is also the perfect finish coat for covering moldless foam or plywood laminations.

    When covering plywood with fiberglass, additional resin will be required to prime the wood as well. For most woods this coat will require about 3 ounces of resin for each square foot of surface. This is in addition to the resin required to saturate the fiberglass.Just to be safe, add 20% more resin to the original estimate.

    The following example will help clarify material estimation as well as cover some aspects of design.

    Construction has been started on a plywood John Boat. The boat is 12 feet long, 4 feet wide at the bottom, each side is 2.5 feet tall, and the transom is 2x5 feet. The ¾” plywood supports the loads, but fiberglass needs to seal and protect both the inside and the outside of the boat. Fiberglass has been chosen over Kevlar to keep costs low. How much material will it take, and how much weight will be added?

    1) Begin by calculating the surface area for each piece.

    12ft x 4ft = 48 sq. ft

    12ft x 2.5ft = 30sqft x 2 = 60 sq ft

    2ft x 5ft = 10sqft


    There are 118 sq ft per layer, and 2 layers will be added to both the inside and the outside of the boat. Next, divide 118 sq ft by 9 sq ft to find the total square yards per layer. This conversion is necessary so the area can be compared to the fabric weights which are listed in square yards.

    118 sq ft / 9 sq ft =13.5 sq yards

    The fabrics under consideration are 10 ounce and 7.5 ounce plain weaves. The fabric weights will be multiplied by the surface area to determine total weight of one layer of fabric.

    10 oz/sq yd x 13.5 sq yds = 135 total oz. / 16 = 8.5lbs/layer

    7.5 oz/sq yd x 13.5 sq yds = 101.25 total oz/16 = 6.5 lbs/layer

    With a 50/50 fabric-resin raitio, the resin will also weigh the same as the fabric.

    Since the boat will only be used near sandy shores, the 7.5 ounce fabric is selected, saving 4 pounds total/ layer (2lbs fabric, 2 lbs resin).If the shore had been rocky, the 10 ounce fabric might have been a better choice for long term durability despite the extra weight.

    2) Caclulate all extra resin and surfacing primer consumption as stated above. The plywood will need a prime coat of polyester resin.It will tak 3 oz per sq ft of surface area to sufficiently coat the surface.

    3 oz x 118 sq ft = 354 oz / 16 = 22 lbs of resin.

    The surface coat will be created by spraying on part # 1041-B Duratec Surfacing Primer. One gallon will easily cover the 118 sq ft with a 12 mil layer of the material.

    To Cover:
    Materials Needed:

    118sqft with 4 Layers (2 inside, 2 outside)
    Resin Primer coat, 55 yds of 7.5oz/sq yd fabric, Laminating resin, Surfacing Primer.

    Type of Covering
    Lbs of Fabric
    Lbs of Resin
    Purchase Quanity

    Primer coat on wood
    1 Gal + Excess of 5 Gal pail Below

    7.5oz Fabric Covering
    55 yds of 38" wide

    Laminating Resin
    5 Gal is economical

    Surfacing Primer
    1 Gal @ 9 lbs
    1 Gallon 1041-B Primer

    Total Composite Weight for John Boat: 83 lbs
  3. Lew Morris
    Joined: Jun 2001
    Posts: 124
    Likes: 4, Points: 18, Legacy Rep: 92
    Location: Pismo Beach, Ca

    Lew Morris Industrial Designer

    Sgray... you are a God!

    many thanks for the website lead, but ESPECIALLY for your very thorough dissertation.

    which of course has lead me to several MORE questions...

    does the use of duratech tend to reduce warpage on light weight parts... say for example; one lam of .75 oz. mat + lam of 1.5 oz. cloth?

    may i assume that the foam you referred to is NOT expanded polystyrene?

    do you operate an FRP production shop per se?

    again, many thanks for sharing your expertise with me.


  4. madi
    Joined: Jun 2004
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Palestine Via Israel

    madi New Member


    Hi All
    I need to build 7m L.O.A fiberglass power catamaran (pleasure craft). the speed is 20 Knots. the hull is semidisplacement. I don,t have more information. Please, one can inform me what the opptimum hull beam and the cross section hull profile for semidisplacment how can show.
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