Fairing the hull with notched troweler.

Discussion in 'Boatbuilding' started by Harrispmi, Dec 9, 2011.

  1. Harrispmi
    Joined: Dec 2011
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    Harrispmi New Member

    I am thinking of fairing the hull space using epoxy compound with notched troweler. hull space measures about 1.8mH and 12meter long.. . I will then fill the gaps after longbed sanding. I ordered a products what is called flexible longboard sander that is about 2meter long so I can do the sanding works with my friend.

    In this stage and method.. what sort of things should I beware of? or... would there be any better ideas for a good finish?

  2. michael pierzga
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    michael pierzga Senior Member

    Make sure that your epoxy fairing compound is exactly the same density for the initial notched trowel application and the following fill application.

    It the two epoxy filler aplications are of different density, you will see a print thru of notched patterns when the epoxy post cures.
  3. gonzo
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    gonzo Senior Member

    That is good advice. This method works for the first coarse fairing.
  4. DGreenwood
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    DGreenwood Senior Member

    That's good advice. Noodling, as it is sometimes called, is a big labor saver if done well. I use it only in areas where heavy fairing is required, then I post cure before final micro fairing. Just tenting and getting the hull up to something over 100 degrees F for a day will usually eliminate most of the print through. It depends on which epoxy/hardener you are using. Call the manufacturer for cook times.
    Use a batten and mark out areas where the fairing depths are greatest. Then size the notches to suit. If you are filling very deep areas the notches should be bigger. Gougeon Bros sell a notched trowel but I make my own, they are more flexible and easier on the hands.
  5. PAR
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    PAR Yacht Designer/Builder

    Unless you have huge depressions to fill, a notched trowel technique is a massive waste of epoxy and filler materials. Most hulls, even old, baddy distorted ones usually just need low and hot spot identification and appropriate steps to correct them. A wholesale assumption that the entire hull needs a layer of goo, is just a backwards approach. It is a fast approach if you have skilled fairers and are paying for their efforts. If not, save the cost and mess of goo and labor, but just fixing what needs it.

    On metal hulls, where welding distortions along the plate seams can be a real problem, this technique can be helpful, but only along those areas. Applying any product with the intent of removing 80% to 90% of it is just wasteful. It's only cost effective when time and labor costs are your concerns and then only useful if you have skilled fairers that can smooth the hull in a single pass.

    All this said, racing hulls, where making the hull symmetrical and extremely fair is a different story. It may be necessary to use many gallons of fairing compound on these, but justification can be accepted on the course.

    Rub your long board over the hull in one direction (only), from stem to stern. Do this at about a 45 degree angle to the sheer. With this done, go back, again sanding in one direction only on the perpendicular to the first passes scratch marks. Stop, clean the hull and look at the scratches. If the hull is fair, you'll have a uniform cross hatched scratch pattern. If not, the lows will be devoid of them and the highs well scratched. Mark the lows and fill. Roughly knock the filled areas and beat down the highs, then again make the two opposing passes with the long board again, just like the previous session. The lows should be higher and the highs maybe still there. Focus on the lows, coming up to surrounding areas and knocking down the highs. A few up and back passes will fair the hull without the need for a 55 gallon drum of fairing compound.

    The short of it is, you have lows and highs, not a whole hull to fair. Fill the lows, knock down the highs and this is a much more economical method then sanding 3, 4 or 5+ times the area necessary by whole coating the hull.
    Last edited: Dec 13, 2011

  6. mastcolin
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    mastcolin Senior Member

    I have never been a big fan of this method regardless of time saving. As has been pointed out you run risk of print through effects due to difference in epoxy, be that in mixing if you use mix-your-own or just in the fact that it cures at different conditions, given no 2 days are same with regards temp and humidity.

    The bigger worry is that you have very small valleys that are unsanded, Depending on what epoxy you are using you are then applying epoxy onto unsanded areas. If you have an epoxy that has issue with amine sweat you are then applying direct onto this. If overcoating times are short this is less of worry but if you are doing coats with days or even weeks between you may be risking adhesion problem.

    How fair are you looking to get it? How bad is the hull? Please tell me you know what you want...and that to get a very fair surface on approx 45m2 (2x12 (loa..or did you mean the lenght of both sides?) x1.8 + transom) is still a lot of filler. A 100% fill we calculate out at about average of 5-8mm. But this depends on build quality.

    If you are going for a almost 100% filled smooth hull just use a piece of angle aluminium or plyboard plank and downwards fill from gunnel to waterline. Mark out the highspots (use as long a piece of angle or board that you can) and hold your hands here and only here. You mention friends. 2 of you can use 3-4m length no problem. If you are on your own just use a shorter length. The tighter the curve, the more flexible you need. (you can vary flexibility by varying angle you hold blade against the surface).

    Zip over with an sander (40-60 is good enough) then do it vertically with shorter bar. Repeat as much as necessary.

    You can plank sand if you want...and to get your money's worth from the plank you just bought :) but if you put filler on with long bar, you can't make it flatter than this. The sanding is only to key and smooth the surface for next application. Regardless of how fair you want to get it if you put it on with a long bar, it won't really get better from board sanding. You'll perhaps make the unfairness less shortwaved but you you won't make it fairer than what you've put on. The sanding with a board is as much as skill as putting it on. If it isn't filled close to 100% you'll be left with unsanded dips cos the board won't touch these. The time you spend on board sanding to find these it after 1st coat is better spent in my eyes just doing as i say and sanding by machine and marking out highs for the horizontal pull and refilling.

    As always PAR offers good advice. As he says you perhaps 1st have to be sure what you are aiming for...and what this entails. Don't be afraid just to bash or reheat the plates that are too high. Of course you can't remove the highs if they are over the frames.

    If the highs are ONLY the frames you can smooth these out just by filling across these. This smooths out the unfairness. It will become more wavy as opposed to just all the frames jutting out as sharp lines.

    To coufuse you further, how are you going to topcoat it? A satin finish always makes it look less unfair. And don't make it a dark colour:) For some reason creamy colours always seem less fussy over quality of filling.

    Hope this helps. It is always difficult to try to explain to someone what to do. One day I will get around to putting all my films and fotos up somewhere..

    good luck

    ps I am busy with 42m J-Class...to be topcoated in black. I am sort of experienced in this:)
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