What's the best glass to use for my application?

Discussion in 'Fiberglass and Composite Boat Building' started by mongo75, Sep 11, 2007.

  1. mongo75
    Joined: Aug 2007
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    Location: Orange County California

    mongo75 Senior Member

    I am taking on one hell of a project- a 25' 1968 Luhrs flybridge that I am essentially building from a bare hull up. My question is, seeing as how I'm using ply for the construction, and glassing over everything to get a good smooth paintable surface, what kind of glass should I use ?

    1-for the cockpit deck?
    2-For the bulkheads?
    3-What kind of coverage can expect to get out of a gallon of resin?
    4-To save money, can I laminate the wood with poly and then bond everything with epoxy, or just suck up the cost and go epoxy the whole way?

    Thanks for the advice, I don't want to buy in bulk blindly, seeing as how I have a mess of 4x8 sheets I gotta cover. I know this is an expensive hobby, but my military salary has a way of slowing down production LOL.
     
  2. alan white
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    Location: maine

    alan white Senior Member

    There is no special fiberglass requirement for what you're doing. Shop for price rather than by specification.
    There are three basic fiberglass types--- cloth, woven roving, and mat. For most all of the jobs you mentioned, 10 oz cloth will be ideal. The other two types have specific uses. It would make sense to but a bit of mat, which is cheap, and use it to build up tabbing. In a layup where cloth is layed into wet mat, the mat conforms to uneven surfaces but presents a smooth surface to the cloth. This eliminates bonding problems due to air trapped between layers.
    Regarding epoxy vs polyester---- much to read if you look around here, but to put it simply, never use polyester as a glue. It's a terrible glue. Never sheathe wood with it. It likes to be laminated to more polyester, preferably all at once. It makes a good hull resin, but from personal experience, I can tell you it will nearly always eventually lift from plywood decks and cockpit soles (three out of the last three boats I've rebuilt for my own use). The sun heats it up, then it cools, then you walk on it, then more heat, etc..
    Use epoxy exclusively for any deck or sole laminations.
    5 gallons of epoxy is available in a big plastic jug, which is the way to go if you want to save money. If I had to guess, I'd say one gallon would yield maybe 50 sq ft of 10 oz cloth on plywood, counting one wetting-out coat, one filler coat, and one final sealing/leveling coat. Others might improve that guess.

    Alan
     
  3. mongo75
    Joined: Aug 2007
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    mongo75 Senior Member

    Alan, thanks for the info. I guess i should have been more specific in my question to save you time- I should have asked what weight, which you answered. I do glass work for a living on USMC helicopters, and have rebuilt two other boats, so thankfullyI'm familiar with all three types. I got another one for ya- what is 1708? I've read about it in rebuilding articles where it was used to build laminate hatches (1708-1/2" ply blocks & cabosil- 1708) is it a cloth bonded to mat or something like that? AND LASTLY LOL- could I get away with only one coat of epoxy to wet out the glass and bond it, and then a light sand and fairing compound to hide and weave, or it is cheaper/easier to just roll on a leveling coat?
     
  4. alan white
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    alan white Senior Member

    I don't know much about specialized glass laminates, especially #1708.
    8 or 10 oz cloth is right for your needs, however.
    You need more than the wetting-out coat of epoxy. A single coat over the wet-out coat is fine if thick enough. The suggestion to apply multiple coats is based on recommedations by those who have an enormous amount of experience in epoxy boatbuilding and long term analysis of their work. Having always followed those recommendations, I cannot say I've had experience doing only one coat. For all the work involved in prep, the final coats take little time, and cost little per square foot.
    Most books showing diagramatically how to glass plywood or substrates of any kind show three pictures. Wet out, filler, and sealer coats. If you're not going to fill the 8 oz cloth, use 4 oz cloth and DO fill it. A bulkhead, for instance, could easily be skinned with 4 oz, especially if it is a manogany plywood (as opposed to fir ply, which ought to be covered with 8-10 oz because the grain telegraphs through otherwise). Bulkheads don't even need glassing, in fact, though they should be epoxied with at least a couple good coats (so why not add glass for a tough surface while you're at it?).
    Cockpit floor (sole) areas ought to get the heavier cloth in any case. Don't skimp there. Epoxy, like paint or anything else, affords better protection from moisture damage if it is thicker. I don't know the exact recommendations for mil thickness, but instead rely on experience and conventional wisdom (several coats).

    Alan
     
  5. PAR
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    PAR Yacht Designer/Builder

    The 1708 referred to in previous threads is a "stitch-mat". Constructed of two layers of 8.5 ounce uni-di (uni-directional fabric) and one layer of 3/4 ounce mat which are lightly stitched together. The 17 means the total weight of the uni-di and the 08 is the weight of the mat.
     
  6. mongo75
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    mongo75 Senior Member

    Alan, Gotcha- Three coats it is.

    Par- thanks on the 1708. In what instances would that be better than just a sheet of uni fabric?
     

  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Using these "combo" types of fabrics are to aid the builder, with a product that provides multiple layers in a single application. I can't think of an application where I'd lay down just a single layer of uni, unless I was working on a particularly heavily engineered structure, having a complex laminate schedule, using differing materials.

    1708 is the same as laying down two uni's and a mat. Because it's stitched together you can lay down this product and wet out all three layers at the same time. This saves considerable time as you don't have to size and apply three different layers of material.

    Most layups use mat as a bonding sponge of sorts, between layers, because of its ability to conform to irregular surfaces. Alone, mat has little strength, but within a laminate can provide a mechanical and chemical linkage between the layers of fabrics, usually better then if the mat was left out of the laminate. In vacuum bagging and other "pressure" laminates, you can skip the mat, if you don't need the bulk.
     
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