Hull fairing questions

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

  1. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    Hi guys,

    I have another question about some issues I've been having with fairing the hull on my new boat. It is a 4-meter open-transom sailing dinghy, built from 4mm okoume plywood. I am at the stage where the all the parts are in place, and it is now time for fairing and sanding. I want to do this before fiberglassing (boat will be glassed inside and out), because there are some surface irregularities, especially on the deck, that need to be faired before fiberglass is laid down, or else I will have to do it on top of fiberglass, and then it might crack over time. For fairing compound, I am using epoxy filled with microspheres, sometimes adding a little bit of fumed silica to prevent sagging. Today I faired the cockpit and the top deck.

    Before fairing:

    [​IMG]

    upload_2020-11-2_21-28-1.png

    First, I wetted out all the surfaces to be faired with unthickened epoxy, and then started applying thickened epoxy, trying to put on a thin, consistent layer. At which I failed completely.

    After fairing:

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]

    I spent 8 hours to get to what you see here. Just mixing epoxy, spreading it, trying to even out with a spreader, adding more to low spots, spreading again... And the result is still horrible. I used up 2 liters (half a gallon) of epoxy just to cover all these surfaces. I was trying to put down a layer as thin as possible, but if I go too thin, then the wood texture shines through, and I won't have a consistent-color surface when fairing, making it difficult to locate low and high spots. And besides that, turns out it is incredibly difficult to put down a thin layer without scraping it off completely. Especially over such a large area. I tried plastic spreaders, steel spatulas, flexible battens... None of these tools give a desired result unless the area to be faired is small and easily accessible.

    This is my first time fairing a hull, and I feel very disappointed at the result, especially considering the amount of epoxy I used up, most of which will be wasted when I sand it out.

    I thought of fairing only areas that are actually uneven, rather than fairing everything like I did. But there are 2 problems with that:
    1. If only some areas have fairing compound applied to them, they will be harder than the surrounding bare plywood. And when I sand, it will be impossible to make the surface fair, because plywood will wear down faster than epoxy fairing compound. Even with microspheres, epoxy is just too damn hard.
    2. It is extremely difficult to spot the low spots and high spots when the plywood is not painted in a uniform color. I tried using portable lights, battens slided over curved surfaces, feeling with my hand, but truth be told, none of these methods gets me close enough to a truly fair surface that would look good after it is painted. Only the paint seems to truly reveal these imperfections... Only it is too late by then.

    So my question is, what am I doing wrong here? How is that fairing supposed to be done without wasting such massive quantities of epoxy, not to mention time? Am I really supposed to cover ALL of the boat with fairing compound? If not, then how do I know where to add it, where the low spots are, before the boat is painted? And if I apply fairing compound only to selected places, how do I avoid having surfaces of different hardness when sanding?

    And most importantly, is there a way to give that plywood a consistent appearance that would show all the imperfections that I need to fill and sand? Some cheap, lightweight material (like paint), but one that will have no adhesion problems when epoxy/fiberglass is added on top of it?

    I mean, I have read quite a lot of boat plans, that include bill of materials... And on boats of comparable size, I have never ever seen such an amount of epoxy listed in the bill of materials that I have used! Surely I am doing something wrong here.

    You have reached the end of a this very long post. Thank you for your patience :D Please help!!
     
  2. jbo_c
    Joined: Jul 2017
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    jbo_c Junior Member

    Are you using a board? If you aren’t, you’ll always just move the irregularities around.

    Also, use an indicator coat to help you know where to work.

    Jbo
     
  3. laukejas
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    laukejas Senior Member

    For small, hard-to-reach spots or curved surfaces, I use a small spreader, like this one:

    [​IMG]

    For big, flat surfaces I use long steel spreader:

    [​IMG]

    What is an indicator coat?
     
  4. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Feel your pain.

    Congratulations. You arrived at the two conclusions at 8 hours that it takes most 8 days or weeks to understand. Fairing can't be done with differential substrates.
    Pre sealing the wood was wise

    The late PAR have the best explation of fairing that I have read. Find and read it if you can.

    Going forward
    Block down what you applied.
    Sheath with glass.
    Add more micro balloon to peanut butter or mashed potatoes consistently
    Spread with a "V" notched trawel.
    Let harden
    Fill the grooves with more mashed peanuts
    Block
    Prime
    Paint

    What kind of sanding blocks do you have?
     
  5. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    It's what you were referring to in your original post.
    It enables you to easily see what needs grinding and what needs filling as you go.
     
  6. laukejas
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    laukejas Senior Member

    I found some info from PAR here. Is this what you meant?

    I have a typical drywall plastic fairing pad which I don't use often, then I have a 5 inch long 1/2" thick plywood sanding block for getting things flat, and I also have a 15 inch 5/32" thick long plywood longboard (flexible). I can make some more boards if needed, but these served well up until now.

    I will try to use that notched trowel method. These are 2 coats, though. Winter is on the doorstep of my workshop... Heating is impossible unfortunately... Today I was working in 9°C (48˜°F) temperature when I laid down that fairing compound :D Poor epoxy takes a week to harden to a sandable state in these temperatures. But I suppose it should harden enough to add that groove filling layer in 2 days or so.

    You mean the fairing compound or paint? I mentioned both in the original post. Because if you meant fairing compound, then I need an awful lot of it just to hide the wood texture. And if you meant paint, then I'm afraid it might prevent reduce epoxy adhesion when I add it on top. Do I need a specific type of paint or something? Can you clarify?
     
  7. jbo_c
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    jbo_c Junior Member

    I meant “are you using a board for fairing?”

    An indicator coat is commonly something like a very light mist of spray paint that you put on before you start with the board so you see low spots easily when you’ve done some sanding. Then you know where to fill and where to stop sanding.

    Jbo
     
  8. Blueknarr
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    Blueknarr Senior Member

    That's the PAR . I don't know exactly where on his site the information you need is.

    Yes more fairing compound. You will need some after the cloth is pit down to fill the weave and wherever it overlaps.
    The 15inch longboard would be my choice for sanding the fairing compound.

    The notched trowel will give you the best chance of an eaven coat of compound. Practice with dry-wall mud on another surface.

    It is better to apply the second coat before the first fully cures.
     
  9. laukejas
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    laukejas Senior Member

    So it is okay to use spray paint? Epoxy won't have any issues adhering to it? Or do I have to sand it out completely? Can this be used on bare wood/plywood as well?

    So it is normal to use so much epoxy for fairing? It's just that the stuff is pretty expensive, and I have never seen so much epoxy in bill of materials of other boat plans. It just feels like there should be a more economic way of doing this...
     
  10. jbo_c
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    jbo_c Junior Member

    Spray paint for a guide coat is common. It should only be a very fine mist, like overspray, NOT like you’re actually trying to color the surface.
    Jbo
     
  11. laukejas
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    laukejas Senior Member

    Okay, I see. Having never done it, I am kind of surprised that a mist would be enough to show these imperfections. Perhaps there is some video or something where this technique is applied correctly? I keep searching on YouTube, but I can't seem to find spray paint used for fairing.
     
  12. Blueknarr
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    Blueknarr Senior Member

    Fairing compound is rarely listed in a materials list. Just like primers and paint. They are concerned "finishing" materials not "building" materials.

    It looks like you could have used much more balloons than you did. More balloons means less epoxy.

    Beware of Urine video of most coat. Most of the ones I have seen show bad techniques which will de-fair a surface. Watch one then remember that the goal is fair (flat) NOT removing the spray paint. Keep working the entire area NOT JUST where the mist coat remains. Focused sanding on a low spot covered with mist only creates a deaper valley.

    I recumend using the biggest, stiffest and firmest longboard you can manage.
     
  13. Barry
    Joined: Mar 2002
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    Barry Senior Member

    [​IMG]FS 42070 E - Shipyard sander by Flexisander | NauticExpo https://www.nauticexpo.com/prod/flexisander/product-29251-307824.html This video really shows the ability of this sander.
    [​IMG][​IMG]
    This is from Flexisand. They also have manual boards that apply pressure more evenly than a a "board" This one looks like you may just use your own grinder to power though I could not find a dedicated unit with the powered and board
    as one unit. I have seen them air powered or electric.
    The board in this case does do corners, though the air type that I have seen was longer
     
    Last edited: Nov 3, 2020
  14. laukejas
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    laukejas Senior Member

    Thank you for these tips. But I'm puzzled: the stiffest and firmest lonboard I can manage? Doesn't that mean that the board will be difficult to flex, placing most of the sanding force at the middle of the board, and therefore creating flat spots there?
     

  15. Blueknarr
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    Blueknarr Senior Member

    Correct. Stiffest possible but still able to flex into the desired curves.
    Too flexible and the board will simply conform to the un-fair surface

    The same with a too soft (squishy) board

    Lots of finding the momma bear's soup

    The hull will conform to a stiffer board, but a flexible board will conform to the hull

    The hull will conform to a harder board, but a squishy board will conform to the hull.

    The larger the board the more the hull will conform to it. But the harder it is to slide around.
     
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