Automotive filler used to fair hull, helpful advice required

Discussion in 'Materials' started by bjviking, Sep 6, 2014.

  1. bjviking
    Joined: Oct 2009
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    bjviking bjviking

    Hi.
    I have built a small 20ft boat using the cold moulded technique. I used polyester resin and chopped strand and roving woven. I then faired this using automotive polyester filler. I now realise thru comment on the web that this was not a good idea as the fillers will absorb water and osmosis blisters will likely occur. If I were to apply a number of coats of Awlgrip 545 epoxy primer, would it seal the fillers and avoid osmosis. Are there any experts out there who can offer advice?

    Yours hopefully.
     
  2. AndySGray
    Joined: Jun 2014
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    AndySGray Senior Member

    There's several types of automotive filler.

    The question is what the bulking agent is?

    some use milled glass fibre (or even chopped strand) and though structurally not as sound, there is little difference between those and laid fiberglass.


    The problem child is at the cheap end of the market where the bulker is chalk - same thing as Talc we use to absorb moisture on our bodies.


    How is the boat to be used - a once a week trailered day boat is not going to experience as much wetting as drying and even the later case will be no issue.

    something which is moored on the water is a different proposal, but then you'll have a layer of antifoul on there and won't notice the bubbling anyway?


    Scrape some onto an off-cut and drop in a bucket of water for a week - see if it is or isnt a problem. :?:
     
  3. gonzo
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    gonzo Senior Member

    Rather than primer, use epoxy resin. If the polyester filler is encapsulated, it should be OK.
     
  4. bjviking
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    bjviking bjviking

    The filler used was upol extra. The boat will be afloat for a few months.
     
  5. SukiSolo
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    SukiSolo Senior Member

    Personally I don't like P38, Upol and the like except for filling keel band screw heads and the like, where it is good. One reason is it carries on shrinking for a fair while and certainly on different substrates ie wood starts to come away after some years. Unlike a polyester glass layup (whatever cloth type) the Upol will not have similar flex so eventually becomes too hard and brittle and starts to lose adhesion to the layup.

    I regularly find GRP build and repairs where someone has used this stuff inappropriately. I usually find out when digging into a 'crack' and it exposes this pink/grey/pale green substance!, the smell, if ground also gives it away. Normally I remove it, ie get a chisel or jigsaw or grinder and replace with epoxy and filler or polyester and cloth. My gut preference would be to remove the Upol and refair with epoxy and fillers such as micro balloons, microfibres etc to suit. Using this technique I have yet to suffer any delamination or problems some years down the line. If you have the choice in that the surface is unpainted, I would be tempted to tear off the filler and put a more permanent solution in place. A good orbital with 60 grit would whip it off prett quickly.

    There are no problems usually with bonding epoxy to a polyester layup. Do not try using the epoxy fillers especially micro balloons in a polyester resin - it simply does not work. There are some other bonding materials about but the mode mentioned above is tried and tested by many, self included.
     
  6. bjviking
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    bjviking bjviking

    Unfortunately I have put too much effort into fairing and finishing the hull now to go riping it all off. I am looking at coating with Interlux Gelshield plus.
    Seems to be a good coating for preventing as well as treating Osmosis.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Gonzo has it right, over coat with straight epoxy, at least two coats, preferably 3. This will lock down what you have, unfortunately, the polyester will still suffer from it's flaws. Epoxy primer or paint will not seal it down, just color it.
     
  8. bjviking
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    bjviking bjviking

    Gelshield experience anyone?

    It seems like the solution is to seal the existing filler with epoxy, then prime and topcoat it. My experience with epoxy is not good. When I tried it in the past I had issues with it curing, whereas I am very comfortable with polyester. I am therefore very reluctant to coat the hull with epoxy in case I make a mess and have curing issues. Trying to resolve that would not be fun.
    What about using Gelshield Plus. It seems designed to prevent corrosion and osmosis and by all accounts is easy to mix and apply. Has anyone experience good or bad with this product.
    Thanks for all the advice so far.
     
  9. PAR
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    PAR Yacht Designer/Builder

    You have to screw up pretty good for epoxy not to cure. It can sometimes have curing issues if applied over alkyd (oil) paints, but not very often. Unlike polyester, you can't vary the hardener/resin ratio by much (some is okay, but try to be precise) and it doesn't make it better in any way, in fact decreases it's strength and other physical properties, including waterproofness.

    Log onto WestSystem.com and download their free "User's Guides", then log onto SystemThree.com and download their "Epoxy Book". These will provide enough of an overview that you'll get it right and may you be able to figure what went wrong on the last batch.
     
  10. SukiSolo
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    SukiSolo Senior Member

    With epoxy, the critical things are temperature, humidity and mix ratio. If you are OK with these you should not get a problem. Right now in the UK we have plenty of time in this weather to get a good coat on in the middle of the day, warm enough to be cured enough before evenig condensation. Even allowing for being in a workshop.

    Do not work in direct sunlight and do not mix too much in a batch, you should be OK. For this type of work you may find a mini gloss roller ideal for application.
     

  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The biggest thing I see with novice epoxy users is, allowing the epoxy to "mass" usually in the mixing cup, which will cause it to kick off in a heartbeat. Spread it out thin once mixed.
     
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