How soon to paint over polyester resin

Discussion in 'Fiberglass and Composite Boat Building' started by burke, May 20, 2017.

  1. burke
    Joined: May 2014
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    Location: Maine

    burke Junior Member

    I will be applying an oil-based Petit EZ primer over a polyester above-waterline FG layup, then a polyurethane paint. How soon can I apply the primer??

    Thanks
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    The application of the paint to the laminate may be less problematic that the primer/finish interaction, one-part polyurethane does not adhere well to alkyd undercoats unless they are fully cured, i.e. odour free. You will have a tacky surface for some time with the laminate (presumably unwaxed resin), but should be good to coat within a few days. Scuffing the surface a little to dull the surface slickness would help, but it will clog sandpaper.
     
    Last edited: May 20, 2017
  3. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Some primers and paints don't bond well to the backside of an unprepped polyester resin surface, so that's another complication.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I have never painted glass laminate with an alkyd primer or undercoat, so don't know how compatible, I've used expoxy primers, acrylic undercoat, polyurethane finish direct, without any failures. Epoxy would be the best way to go.
     
  5. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Depending on the exact resin, it will take a while for the surface to be come tack free enough for some paints to stick, others stick well to the still under cured surface. This is were type of paint and timing come into play. Epoxy would stick well to the fully cured surface, but may fail if it's still tacky.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    From memory, I have painted epoxy primer over a few days old laminate, with no nasty consequences. Mind you, here in sunny Queensland is climatically rather different to Washington State ! More like Florida.
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The laminate I speak of above was isothalic polyester. Done that on a few occasions. Still sticking.
     
  8. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Yes, various paints used in various ways will stick, not all will though.

    ISO typically cures fairly well on the back side after a few days, but depending on catalyst level, ambient temp and humidity, the surface may stay sticky much longer, not all paints can deal with this. I've seen paint stick for decades, and I've seen paint peel off in a few weeks. For about 15 years I painted the backside of large high pressure water filtration tanks, many per day, we never had a problem with the two part paint with no primer, they sat out side for many years (decades) and none failed.
     
    Last edited: May 21, 2017

  9. burke
    Joined: May 2014
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    Location: Maine

    burke Junior Member

    Thanks all,

    Your first comment: I called Petit over the winter to ask if I could use a single part polyurethane or a latex enamel over EZ Prime Hi Build Undercoater 6149. They said yes. Just checked the Data Sheet and it said single or two part enamels OK.
     
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