Hull fairing questions

Discussion in 'Boatbuilding' started by laukejas, Nov 2, 2020.

  1. laukejas
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    laukejas Senior Member

    Well what exactly is a primer in this context? I have inquired to all my epoxy suppliers for epoxy primer. No one has even heard of such a thing. They tell me to apply paint directly to epoxy, and it will stick (in my experience it does). What is a primer here, then? Perhaps you mean just a thin layer of paint? The only commercial fairing compounds I can get here are polyester-based. They don't work well when applied on top of epoxy, I heard...

    Please correct me if I'm wrong. I don't get what boatbuilders call primer.

    P.S. I figured that perhaps by primer you meant specialized paints that are divided into undercoat (primer) and topcoat? The only paints I ever used have no such separation, and are sold in a single can. The only paints with that separation into undercoat and topcoat I ever saw are one of these premium yacht paints that are extremely expensive; I try to stay away from them.
     
  2. Blueknarr
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    Blueknarr Senior Member

    Primer is a specific paint category. They are usually cheeper than top coat paints and are formulated to by applied thicker and sand easier then top coat. They don't have uv resistance so must be over coated with a proper paint.

    Use extra coats of top coat paint if that is what is available.

    But proper primers do better for the initial coats than top coat paint would
     
  3. wet feet
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    wet feet Senior Member

    You should not expect miracles from peelply.It's function is mainly to absorb the amine blush that epoxy leaves so that subsequent fairing and filling can adhere to the surface with no extra preparation.Before applying paint you will need to sand off the worst of the pimples and ridges or you will be forced to add a lot of filler to disguise any blemishes.You may break into the strands of glass but unless there were ridges of obstacles under the glass you shouldn't seriously weaken it or go very deep.When the warmer weather comes you will almost certainly need to fill the weave of the glass with a very thin screed of filler,I use a mixture of microballons and epoxy for this.I apply it with a smooth plastic spreader just as a car paint specialist would.It will need a little sanding and then a coat of primer.Such stuff does exist.The negative aspect is that it isn't cheap Marine Epoxy Primer Paint – 2 Part Kit | Supreme Paints & Coatings https://www.supremepaintsandcoatings.com/boat-paint/4-bottom-protect---high-build-epoxy-primer-2-part-kit or you can use a 2K car primer which costs a bit less 2K High Build Primer https://www.specialistpaints.com/products/2k-primer .It is very similar chemically to two part marine paint and also needs to be used with good ventilation as the isocyanate hardener is pretty toxic.If you are not spraying there won't be quite as much airborne paint.You may still find a few pinholes and have to fill them.Then you sand until super smooth and apply the gloss finish of your dreams.It isn't quick or easy,but it can be done.
     
  4. fallguy
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    fallguy Senior Member

    sorry BK, but you forgot something with this post!

    Topcoats cure hard as hell and impossible or very difficult to sand and fair back to..

    Epoxy primers are many.

    Epifanes
    Systems 3 Yacht Primer
    Interlux
    EMC
    Alexseal

    ...all have epoxy primers to prep for two part polyurethanes

    But paint is a system. The primer and topcoats are matched; usually.
     
  5. laukejas
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    laukejas Senior Member

    Okay, got it. But unfortunately, I have no affordable option to get a primer for my boat. None of what you guys are mentioning can be purchased where I live. Regular epoxy is pretty cheap around here. But that primer paint stuff is around 8 times more expensive. And epoxy primers separately aren't sold at all. What can I use instead of primer? What if I used the same microballoon-filled epoxy mixed with a strong pigment to give it a good color?
     
  6. rwatson
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    rwatson Senior Member

    Is a good quality waterproof "wet area" ( Bathroom) can of Undercoat cheap in your part of the world?
    I have used that on Epoxy in a canoe with very good results over many years.
     
  7. laukejas
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    laukejas Senior Member

    Yeah, I can get quite decent paints cheaply around here. But any paint that comes with a primer is in entirely different price range. And regular household primers are definitely not meant to stick to epoxy.
     
  8. wet feet
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    wet feet Senior Member

    Can you get 2K car paint?Its better than house paint because it has to cope with extreme temperatures and UV light.There will be primers for it.As posted earlier,it needs good ventilation and a charcoal filtered mask is a really good thing to have.On a technical point,why don't you think some forms of paint will stick to an epoxy surface if the epoxy is free from contamination?
     
  9. laukejas
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    laukejas Senior Member

    I used 2K paint on one of my earlier boats that I built for sale, so the price wasn't the issue, customer wanted the best. But that paint alone cost as much as the rest of the boat, including oars, rigging, hardware, etc. So for my own boats, I tend to stick with single-part paints.

    I believe that most paints should stick to epoxy; I just meant that non-epoxy primers might not stick. Most primers I can find around here are polyester-resin-based, and I believe that polyester doesn't stick well to epoxy for some reason, isn't that right?
     
  10. Blueknarr
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    Blueknarr Senior Member

    Correct polyester doesn't stick well to new epoxy.

    Most of the auto industry high build primers are polyester based.

    A high build primer would yeald the best results.

    But if none are available then extra coats of top coat paint can yeald acceptable results. It usually take greater effort at higher cost than using high build primer.
     
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  11. laukejas
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    laukejas Senior Member

    So if I use a top coat paint instead of a primer, and it reveals that I have some surface defects, like pin holes or something... Then what? I remember reading that epoxy doesn't stick as well to existing paint as it would to previous layer of epoxy (even if fully cured), and if for example the top coat paint reveals that I have a small depression or a hole in the surface, it would be very difficult to sand the paint in that hole so that filling epoxy could stick to it. Am I wrong?
     
  12. Blueknarr
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    Blueknarr Senior Member

    You're not wrong. This is why high build epoxy priming has been recumend many times to you.
    You keep saying that it isn't an adorable option.

    Plan B. Skip the priming and use a lot of paint.

    One can drive from Moscow to Paris via India. It just takes longer with a higher fuel bill.
     
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  13. laukejas
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    laukejas Senior Member

    Fair enough. Sorry. I'm just trying to find a solution with very limited means. I wonder though, what makes epoxy primer different from regular epoxy resin? Apart from being thickened and pigmented. I'm just wondering if perhaps I can cook up my own epoxy primer from that regular resin with the right combination of additives.
     
  14. fallguy
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    fallguy Senior Member

    Epoxy primers contain pigments. If you don't use a pigmented primer; it jist takes more coats of expensive, hard as hell to sand topcoats.

    Epoxy is far too viscous to use as a paint. Guys use it to weave fill all the time. That is different. The mil thickness is much higher; drips far more likely, plus at $100 a gallon; there is a diseconomy for less coverage at higher cost. You don't have to use epoxy primers, but a primer is a must.
     

  15. Blueknarr
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    Blueknarr Senior Member

    Fallguy tried last year ish.
    Ask him how bad it went.

    Yes you could
    BUT
    The additives are way harder to get than the mixed primer.

    The additives are ground two orders of magnitude smaller than what you have been adding. There's also additives to make it flow smoother.
    And thinning solvents. It is basically paint that uses epoxy as it's binder. Think of it as boxed cake mix vs flour (epoxy).

    I suggest a thin final fairing coat.
    Block sand
    A coat of neat (unaltered) epoxy
    Block sand.
    Paint
     
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