But first, a word about polyester resin

Discussion in 'Fiberglass and Composite Boat Building' started by gaffer, Nov 29, 2008.

  1. gaffer
    Joined: Aug 2008
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    gaffer Junior Member


    A word about polyester resin

    The resin wont "soak through" it will only go into the ply as far as the first ply,,and when using poly,you should first cut your wood and pre-drill any holes,, then put 2 coats (3 is better) of resin on,, then the next day take 36( beginners should use 80grit) grit and go over everything ( carefully,,your just "scratching it up to bond),, then put your glass on.and in all practicality,, after your first 2 coats,, the next day go back and get any "dry" spots where the resin soaked in more.

    About polyester? Have any of you got a good word for polyester.
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    No... not with wood, ply or anything else except glass..
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    Not for wood or ply

    Polyester resin hardly glues to itself and it's not waterproof. Yes polyester soaks water...Add that wood and plies contain phenols that inhibits the curing of the polyester so the result has not any peeling resistance.

    For those that wants absolutely use polyester resin over plywood there is the following method:

    1- scratch the surface of the plywood
    1bis- 3/4 inch holes every 8 inches in all the surface of the plywood is a plus. The holes will be filled later with resin and mat, making small bridges between the 2 polyester fiberglass coats.
    2- soak the surface with styrene added with 2% of accelerator (generally cobalt)
    3- let it dry a few hours
    4- re-soak the surface with isophtalic polyester resin diluted with 25% of styrene and catalyzed with 2% of the total weight (resin plus styrene) catalyser, generally a peroxyde.
    5- when almost cured but always "poisy" one light coat of isophtalic polyester resin catalysed at 1%.
    6- when almost cured but always "poisy" you can put the fiber and wet it.
    I almost forgot: use isophtalic polyester resin with NO PARAFFIN. Only the last finishing mat will be wetted with paraffined resin so you wont have a poisy surface.

    The purpose of a so complicated system is to obtain that the polyester cures fully even in contact with the phenols of the wood and to get chemical bonds. RESULTS ARE NOT GUARANTEED.

    I think that a good epoxy is simpler, faster and finally cheaper with better results...

    I'm rebuilding a 36 feet Cigarette for a dive shop: every piece of plywood is soaked of water and rotting. The polyester/fiber peels with little effort from the plywood as it has never bonded. We are going to stay with a bare polyester hull and I'll rebuild a rationally designed supporting structure with BS1088 marine meranti plywood, chiapas cypress wood and epoxy resin.

    If you want I can upload some pics of this boat to illustrate that polyester and plywood do not work. I have worked in boatbuilding since 1970 and I have never seen a plywood/polyester last more than 5 years.
     
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  4. PAR
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    PAR Yacht Designer/Builder

    In a chemical bond situation, polyester is capable of a fairly good bond with itself, little more. In a mechanical bond, the surface must be very well "toothed" up. This bond is moderate on itself and poor on wood. There isn't a debate or running argument about this, the jury has been long in, on the physical attributes of polyester resin systems and wood. The two dimensional molecular relationship, of the chemical parts of catalyzed polyester resins, is the primary reason for this and though you can improve the mechanical bond a bit with technique, the physical properties just have limitations. It's a modulus of elasticity differential problem and not solved with staples, exposed nail heads, extra mat, holes, 24 grit abated surfaces, unique mixtures or any other cute "special" techniques.
     
  5. Ilan Voyager
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    Ilan Voyager Senior Member

    I agree with you and truly I do not understand the obstinateness of a lot of people, even professionals, wanting absolutely to use polyester resin on wood, and specially cheap exterior plywoods. This composite has bad characteristics, it is no so cheap as it implies a lot of resin and tons of fiber, and doesn't last. About bonding polyester resin on an already cured polyester surface, the lone way is to sand until the firberglass and make bond the resin on the exposed fibers...

    For example, on working boats decks a good firemat layer gives better results to get thickness for a weight not so far from a fir plywood. I gave the "recipe" to illustrate the work and materials implied by the composite plywood/polyester with bad results, in front of the simplicity of a epoxy resin/plywood which has a proved record.
     
  6. PAR
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    PAR Yacht Designer/Builder

    As a molded product, shot in one go, well infused fibers, with a reasonable resin fiber ratio, then it's suitable for pleasure craft. The problem lies with the engineering of the structure, rather then the polymer itself. The cost management team gets their hands on it and asks if this or that can be removed or replaced and still withstand the one year warranty.

    Some very well engineered polyester yachts have been done. The dark days of the 70's and 80's put an end to the well engineered production boat. It's only now being revised as the industry has made material changes, which required new engineering and structure analyses. This has produced better, stronger yachts for the most part.

    In the end, Ilan is correct, you don't save that much money, because the laminate needs so much more effort and bulk to get similar strengths to other resins. Without question it's use on wood is a short term option. Anyone considering such is building a "proof of concept boat".
     
  7. ondarvr
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    ondarvr Senior Member

    Polyesters are what I do for a living and for the most part I agree with you.

    I never recommend polyester over plywood as a method of construction, it just doesn't meet the desired strength or longevity that epoxy can provide.
    One thing about people or customers that insist on using polyester is they frequently want to take short cuts in other areas also. So even if the project could be done with reasonable results if done correctly, it has no chance of succeeding with the methods used and many times epoxy wouldn't hold up with the poor methods and workmanship used either.

    Much of the bad rep polyester has is due people building boats or other items with resins that were never designed for the environment the finished product is used in. So when I hear the polyester won't stick to anything and it's not water proof comments, I understand where it's coming from. If you choose the correct resins for the job and then use them as recommended, it's possible to get good results. I'm not saying epoxy doesn't do a better job, just that many times polyesters aren't as “horrible” as many people think.

    We sell a huge number of resins, most formulated for a specific use, but the best products are not the big sellers, it's the lowest cost formulas with the inferior properties that are in demand. These products are less water resistant (nothing is waterproof, only different levels of water resistant), brittle, have poor bonding properties, etc, these are the big sellers.
    The retail customer normally doesn’t have enough knowledge about the products to even know there could be an issue with a low quality resin, this results in builders selling on price. Many builders start out with good intent and then when they’re at risk of going out of business they start looking at the cost of materials as a way to reduce costs and be competitive.

    Another problem with polyesters is their very forgiving nature, you can abuse them in many ways and they still get hard, they may not be cured correctly, or have even near the physical properties they should, but the part looks OK as it goes out the door. These products last a few years and then fail, when they do the resin get’s blamed.
     

  8. Ilan Voyager
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    Ilan Voyager Senior Member

    Yes, polyester is a good material when used as it is intended to be used. You are convinced when you see 30 years boats almost as fresh as new, for example the sailboats made by Wauquiez in France.

    As any material polyester has its strong points and weaknesses. Secondary bonds must be done with great care and gelcoat, or epoxy finishes sealers solve the little problem of water absorption. Nothing difficult.

    I just emphasized that wood and polyester is a bad combination, even done with care. I never have seen one lasting more that five years without having troubles. Mine sweepers were made, under very stringent norms, by a very good shipyard in the beginning of the seventies; after ten years of navigation it was a disaster; all the parts made in plywood/polyester were delaminating.

    About quality and price, it's alas too common that people go immediately to the lowest price. I see how the local shipyards, here in Mexico, use the crappest orthophtalic resin with too much MEKP on humid fiberglass cloth, the results are astonishing after a while. The obsession of the lowest price is very expensive at the end...
     
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