Hull fairing questions

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

  1. jehardiman
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    jehardiman Senior Member

    Though there is a lot of good info in the above posts, and you may be too far down the road for this to help now, a long time ago an experienced professional boat builder once answered my same question with a question..."Do you want to sail or do you want to sand?".
    Getting a surface that will take a gloss black finish is a MASSIVE amount of work, probably more hours than the construction. Most commercial wooden working hulls are finished flat white, just to avoid all this extra work. For cold molded racing yacht builds this was his method.
    After sheathing/saturation, longboard the whole hull with medium grit.
    Shoot the hull with a high build primer.
    Longboard again with a finer grit, the idea here is to show the highs and hollows
    Use the high build primer again in just the hollows
    Longboard and prime again, cutting down the highs (but not through the sheathing) and filling the hollows.
    Do this until you are done sanding....the finish you want will dictate.
    Seal coat
    Rough it up with very fine grit by hand
    Paint.
     
  2. jehardiman
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    jehardiman Senior Member

    A wooden sanding longboard should have peg handles on each end. This will allow you to camber the board without having to press into the hull. Remember, you are trying to sand the hull, not file it.
     
  3. laukejas
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    laukejas Senior Member

    Well, I may be damned, but I do want the boat to shine gloss with no imperfections as far as the eye can see. I don't mind the elbow grease. I don't mind the sanding. It is winter after all, it's not like I could be sailing instead. The only issue is, epoxy takes just so damn long to harden. Even at normal temperature, it is at least 24 hours before you can sand it. And in my temperatures, a week at least. That makes me want to fill all the way so I don't have to re-fill after sanding, but it would be very wasteful, as I don't know where to fill. Are there any other filler compounds that I can use instead of epoxy, something that cures in an hour or two? Polyurethane looks like it, but it doesn't stick to epoxy-covered surfaces. And I have plenty of them. Is there anything else that I can safely use without fear of delamination?

    Also, you guys keep mentioning primer. I am assuming epoxy primer. Why exactly do I need it? How does epoxy primer differ from regular epoxy? On my previous boats, I used no primer, just regular epoxy, and then painted directly on top (with proper surface preparation, including amine blush removal). Never had any problems. I understand that with regular surfaces and paints, primer is like a compatibility layer than ensures paint adhesion. But it seems to me that paint sticks to regular epoxy just as well. So why do I need primer?
     
  4. Blueknarr
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    Blueknarr Senior Member

    Now that you are covered in epoxy. It would be very difficult to change.

    I'll repeat my warning that there will be more faring after glassing.

    Is there a cold hardner available for your epoxy?

    Priming is not just for adhesion. You could go straight to paint.
    You won't NEED to primer but you will WANT to.
    Primers are usually cheeper than paint.
    Primers can usually be applied thicker than paint.
    Primers are usually easier to sand than paint.
     
  5. jehardiman
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    jehardiman Senior Member

    High build primer for marine is generally a polyester or lacquer based fill paint that contains small particles (i.e. high solids) that add depth to the paint, often making a single dried layer several mils thicker than regular paint. It is specifically designed to flow well and hide surface scratches left by sanding. And all the other stuff said by Blueknarr.
     
  6. laukejas
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    laukejas Senior Member

    Yes, I am aware I will have to fair after glassing as well. I do try to make the surface as smooth as possible before glassing to minimize that second fairing. I want to have as thin layer as possible on top of the glass. I worry that having a thick layer might lead to cracking later on.

    Yes, there is a cold hardener available. I am using it :D It is VERY cold out here. And it will get colder. I am working partially outdoors, and there are thousands of holes in my brick-walled workshop. Unfortunately no way to insulate and heat it at this time.

    I will check with my epoxy supplier if he has any primer epoxy. My regular epoxy costs around $14 per liter, or $53 per gallon. That's quite expensive in my part of the world.
     
  7. Blueknarr
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    Blueknarr Senior Member

    $50 gallon is common in the US as well
    A trick I have used once the epoxy is set is to lay electric blanket on it. Cover with more regular blankets if possible.

    You might also wish to install all the deck hardware before final fairing and paint. It will be easier to fix relocated holes and scratches now.
     
  8. fallguy
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    fallguy Senior Member

    I have some advice. It differs a bit.

    1. Get a 3/4x3/4" clear straight board long enough to cover the deck midships port to star. This is a batten. Lay the batten on the deck and look for high spots and low spots. Mark the low areas with a plus sign with a sharpie. Make the highs with a minus sign. Do not use a red sharpie, black is fine.

    2. Do not use those tools you have been using. They are terrible! Fairing tools must be stiff. The only time you want flex is on round hulls. I don't see a round hull. I see developable surfaces. That is a flat surface with some curvature.

    I use a 12 & 14" concrete trowel and I use a one handled and a two handled angle iron. It is aluminum and the one is like 24" long and the two is about 40" long. These tools allow you to make long, sweeping movements. Your tools are the issue; you are making small, staccato movements.

    3. Each fill should be a little less than the last. The fill needs to be sandable balloons with a bit of silica. It looks like straight fumed silica?

    4. I use homemade mix for the beginning. Then I swotch to commercial compounds at the end. They feather out better.
     
  9. fallguy
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    fallguy Senior Member

    Also, I would estimate that boat of yours would take between 64-128 ounces of epoxy plus fillers.
     
  10. laukejas
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    laukejas Senior Member

    Good idea with the batten. Definitely will try that.

    Fairing tools must be stiff - how stiff? The longboard should still be flexible, right? If it is too stiff, it will create flat spots at the middle where force is the greatest, right?

    I already used up way too much epoxy for filling this first layer... Perhaps if I had used a notched trowel it wouldn't have been this bad. I 3D printed several notched trowels with different teeth size, because commercially available ones don't offer much choice. I will try them out next weekend and see which teeth size prove the optimum fairing compound thickness.

    [​IMG]
     
  11. Blueknarr
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    Blueknarr Senior Member

  12. fallguy
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    fallguy Senior Member

    2E1CD9CA-A7A2-4CDF-98C5-72ABAEC76797.png
    Respectfully, I disagree on drywall tools. Agree his spreaders are flexing too much. I had a professional drywall guy help me on my boat. The first day there he said the drywall tools were to flexible to achieve needed results because the epoxy is far thicker than water used in drywall work. Concepts are similar, but boats are not flat walls and he struggled.

    This fellow is all confused about fair lines.

    The deck is curved, but it does not appear to be compound curves. If he fills the deck the wrong direction, it will never flatten out.

    green is good, red is bad
     
  13. fallguy
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    fallguy Senior Member

    By the way, the arrow direction for the green can also be on a 45, but the tool must remain in the x plane..not 45...

    Personally, the notched trowel method can work, but the fill demands are way to high. Had he used a straight angle iron like the green arrow shows to start; he could have labelled the infill needs woth a sharpie. i.e. 1/32" here, a highspot is 0", a 1:16" there, or a deep hole 1/8" there.

    Let's say he uses a notched trowel now and the deck is 4 square yards. A 1/16" fill is 324 cubic inches of compounds! Let's say 3 cuin per ounce of epoxy. That is 100 ounces epoxy. A horrible idea.

    Sand it out in the green arrow method or a 45, but never red. Then use stiff tools to fill. The tool stays in the x plane at all times or you'll be doing it next year.
     
  14. wet feet
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    wet feet Senior Member

    At the risk of upsetting a few people,I have to say that you have done the job so far in a way that will cause a lot of work-both now and in the future.I suggest you sand the bumps out in any way you can and get on to the glassing because that will be at least as much work to fair and finish.The problem is that the temperatures in northern Europe won't really be suitable for another six months or so.Which means you will be lucky to get the boat complete and sailing next summer.High build primer is a preparation for a paint finish and ought to be applied after the structural work is complete and the glassing is for the purpose of adding strength isn't it?Having had a dinghy with a black deck my advice is don't do it on this boat as the black can be unbearably hot in the summer sun.A black hull isn't so bad in this respect but if you sail in salt water,you need to be prepared for the hot surface to promote the crystallisation of salt on the surface.Black is also very unforgiving of any surface blemishes and will require either a huge amount of work or an acceptance of slight imperfections.Its also the colour of several finish coatings used in the preparation of plugs for moulding precisely because it shows any blemishes.

    When you ready to finish the outside a suitable fairing block can be made from Dow styrofoam perhaps 400mm long and 60mm wide and 12-14mm thick.It will be flexible enough to follow a hull shape and still able to wear the high spots away.I advise wearing gloves,at least for the bulk of the work as you will have no skin left on your fingers by the end of the process.There are some useful videos of the fairing process being done by professionals on youtube-taken when superyachts are being prepared for a wonderful finish.It takes time and effort and the cost of epoxy filler and other consumables does add up.

    At the risk of causing yet more upset,why are you glassing the hull and deck anyway?Thousands and thousands of sailing dinghies were built in the boom years of dinghy sailing with no glass covering and they worked well.Have you looked at the build process for the Seggerling and the Phantom? They are similar in concept and not glassed as far as I know as it would add a chunk to the weight.Admittedly,a covering of glass will resist punctures a bit better than plywood,but as you note epoxy is horribly expensive in Europe and not exactly cheap in the USA so it makes sense to use it only where it needs to be.
     

  15. waikikin
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    waikikin Senior Member

    Hi Laukejas,

    I've done quite a bit of this. One of the best fairing tools for convex surfaces is your wood plane, make sure it's genuinely sharp and keep on this aspect, & even use a light smeer of wax on the base(floor or mould release) & polish that off well. Use a bit of black spray paint very lightly misted on as your guide coat. The concave surfaces in the cockpit will be tougher but just lay into them with the RO but float it. On the corners it's good to fair out to a sharp chine so the true line is found before using a consistent process to round over. Really the timber work should be close to fair and the putty mix just used over fastenings and some joins only if they need it. Timber is strong, the glass sheathing "stronger" but the lightweight compound weaker... so it should be on the outside.. too bad, too late:)
    Sharp & ground tapered broadknives in varying widths are your friend in application, go to a plasterers/drywall supply & dont skimp- even application save fillcoat and time so therefore money.
    If there's an issue with curing in cold weather you need a plastic tent so you can cook to a temp before application of glass or filler, then apply. Just a small fan heater of 2200 watts dose a great job in assisting cure in cold- just remember that when on a porous substrate such as timber you need a falling temp for application.

    Have fun, all the best from Jeff.
     
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