marine primer - paint chemistry question

Discussion in 'Materials' started by Anatol, Sep 21, 2017.

  1. Anatol
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    Anatol Senior Member

    I'm getting to the point in my glass/epoxy over ply project that I need to think about primer. It'll be alkyd enamel out the outside. I want a high-build effect as I'm not vacuum bagging and do not want to have to sand off glass I've laid. I am on a tight budget so I'm looking for cost savings, even if it means extra labor. My questions -
    1. what kind of (mineral?) filler would be appropriate for adding to a primer to increase 'build'? Presumably nothing hygroscopic like talc. There would be a clear limit to how much to add before compromising other qualities. 10% by weight?
    2. I can't afford west marine prices. What, if any, domestic primers might work, with good adhesion? I looked at oil based zinger coverstain, but is is not recommended for immersion. How much of a problem would this be, if its over glass and has a couple of coats of enamel over it?
    3. What about other kinds of industrial primers? Some of those gray metal primers are tough, but are they counter indicated for over epoxy?
    any comments appreciated.
  2. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Are you hoping to use the primer as a faring?
    True epoxy fairing compounds work much more better and cheaper to boot.
    There is a maxim thickness for paint usually measured in thousands of an inch. Using primer as fairing will often exceed this limit.

    Generally speaking, the more resent in paint/primer the better its adhesion and cohesion - the more solids (aluminum or titanium oxides) the thicker the coat and easier to sand, because its cohesion is less. In the marine environment adhesion rules.

    Zinzer primers rock! I used them for twenty years. They know their stuff, so don't mess with their formula.

    Why alkyd and not 2part urethane?

    I'm partial to duratec products.
    Last edited: Sep 21, 2017
  3. ondarvr
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    ondarvr Senior Member

    There's nothing cheap about fairing primers if you want them to hold up in a marine environment. Use the right stuff or you'll regret it later

    You can lower the cost by buying the epoxy and correct fillers and mixing it yourself, but don't try creating your own blend of cheaper fillers, use the correct ones.
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    You may need to sand just a little, or fill a lot more. A high-build epoxy primer will have good adhesion, sands well, and can take a variety of finish coats.
  5. jorgepease
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    jorgepease Senior Member

    I think you will benefit from both fairing and a high build primer.

    Get a 4 gallon bucket of microballoonsFL-PH400 4GAL 4 LB $68.50
    5 qt of cabosil ...FL-AER050 5 QT 8oz $7.60

    You can use the West Epoxy if you want, it will go a long way

    ... mix it up DRY real good so you have a consistent mix ... and then scoop it out as you need it, power-mixing with epoxy - not hand mix- with x amount of epoxy to suit thickness needs. Power mixing will give you a very creamy mix unattainable by hand mixing but do it at slow speed, you're not trying to whip it up and entrain a bunch of air.... apply it with a notched trowel, then apply second coat within about 8 hours and you won't have to sand between those first coats.

    Then break out your flat board and start sanding for subsequent coats washing with water in between to get rid of all that dust. This will get you the farthest on the least budget.

    Micro Ballons make sanding easy, Cabosil lets you feather better and hardens the mix good. If you don't use the cabosil you will clog your sandpaper fast.

    Have Fun!
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There's no such thing as a "fairing primer". The best you can do with a primer, even a high build one is to fill very slight imperfections, in the regularity of the surfaces. Extremely shallow depressions and scratches is about all you can expect from any primer.

    Adding things to primers can be done, though you do need a good understanding of the chemistry involved or you'll ruin the peel strength, of the cured film. Typical additives would be silica, mica and other flake like particulates or bulking agents, plus some binders. If you add much of anything to a high build, you will likely ruin its ability to bond well, as this stuff is usually maxed out, but regular paints and primers (thinner stuff) can accept some additional filler materials. Ultimately, you'll need to perform some tests on various formulations, to see how much of what you can get away with.

    Lastly, if you're looking at West Marine for pricing, you're looking the wrong place. These outfits are full up retail and online sales are far less costly. Paints (generally) aren't an area where you want to cut costs, because you'll end up with what you've paid for, a cheap film with marginal abrasion resistance, low gloss retention, difficult repair and touchup, low UV resistance, etc. On the other hand, if you apply the coating with a wire brush, you can consider Ace Hardware brand paint as the best/cheapest available.
  7. Anatol
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    Anatol Senior Member

    Hi all
    thanks for your replies. I do understand the difference between fairing and primer.

    Blueknarr, thanks for encouragement on zinser products. But the 'no immersion' advice still worries me.

    Jorgepease, thanks for your advice on mixture and mixing. When you powermix, do you use a paint mixer in a drill? I'm surprised by your recipe as I have experimented with combining microbaloons and fumed silica as a filleting material and it was a sister on vertical surfaces. They thixotropic quality of the silicon combined with the rolling of the balls produced a lumpy oatmeal effect, with big clots rolling over each other. Difficult to smooth out in corners! As a side inquiry - Can anyone suggest a good tool for putting a smooth radius back on that mess? My best guess is a 'cup' wire brush on a die grinder.

    PAR, could you elaborate on "other flake like particulates or bulking agents, plus some binders".
    btw, I was not suggesting I should add anything to a high build primer, I get it that this is already maxed out.
    also, I avoid West Marine, I just can't afford it, I do buy online but in CA some formulations are banned.
    Hence my question about 'enhancing' an exterior domestic or industrial product...

    So maybe I should change my inquiry to ask about fairing mixtures. Qualities should be lightweight, sandable, with a good bond. For me that says epoxy with microbaloons and a sandable mineral filler (Chalk? Talc?). I am wary of fumed silica for reasons discussed above. I assume this mix can and should be very thick, but when it gets that thick, its very difficult to apply (smoothly, thinkably, effectively) - so what is the trick guys?


    PS -this page offers an interesting summary of mineral fillers. I found it informative, though it makes no reference to marine uses.
    Functional Fillers and Specialty Minerals for Plastics
    Last edited: Sep 22, 2017
  8. Blueknarr
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    Blueknarr Senior Member

    California's coating restrictions allow for maritime use exemptions. But try to explain that to the local supplir. Easier to maintain shipping drop in Reno. Vegas for so-cal.

    The absolute best practice for spreading fairing compounds:
    Dry-wall topping mud. Totally useless on a boat but cheep enough to practice by skim coating your garage walls.

    I like using tongue depressors to spread bog into inside fillets. To sand, try folding paper around a short length of tubing.

    Good luck.

  9. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    There are high build sprayable fairing primers, both polyester and VE.

    They can be applied at 120+ mils if needed. They're designed for this exact purpose, plus they're epoxy compatible. I wouldn't recommend them for this exact application though.
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