Quick question regarding corner radius for proper glass adhesion

Discussion in 'Boatbuilding' started by OrcaSea, Jan 13, 2015.

  1. OrcaSea
    Joined: Oct 2014
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    OrcaSea Senior Member

    I am getting close to applying glass (w/epoxy) to my stripped deck of my 16' restoration.

    When I obtained the boat there were cracks in the fiberglass where they original builder forced the glass around 90-degree corners. I understand that glass doesn't like sharp corners.

    My question is, how much of a radius do you need to prevent the stress cracks and ensure proper (and easy) adhesion? Simply breaking the edge to a rounded surface with a sanding block? A 1/4" radius?

    There are fasteners (screws) with relatively short edge margin to the edge of the deck, so I don't want to take too much material off and weaken them or risk hitting them with a router, I just want to work the material enough to address the issue and no more.

    Thanks!

    Curtis
     
  2. SukiSolo
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    SukiSolo Senior Member

    Do we assume external corners? or internal? If the former a small rad is ample, max 3mm if you use a light roving say 200 gsm. You may need a couple of layers. The worst stuff is usually heavy CSM which will never go round a sharp corner!. Sometimes a local thin tape first is a good way to get extra 'grunt; where you need it.

    For internal, put a fillet in there first ie epoxy microfibre/wood dust. Then put the glass onto that after sanding, or even on it wet.
     
  3. OrcaSea
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    OrcaSea Senior Member

    Thanks, Suki, I forgot to mention it is an outer corner.

    I do have some 2" and 4" tape that I had bought when I thought I was going to try and repair the existing glass, so perhaps 4" tape, centered, and then scarf the edges?

    I have seen a couple different approaches to blending/scarfing overlaps - one is to simply scarf the edge and the other was to sand off the edge of the tape (the proper name of the edge/containment weave escapes me) and use fairing compound to blend. Any opinions on which is best, or is it 6 of one, half-dozen of the other?

    Thanks!

    Curtis
     
  4. PAR
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    PAR Yacht Designer/Builder

  5. OrcaSea
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    OrcaSea Senior Member

    Thanks, Paul!

    I've actually looked up your website, drooled over your designs and read most of your material before - well written and informative, thanks!

    I have two questions, if I may:

    Just to be clear, all blush must be removed even between laminating coats of epoxy, correct? I assume the blush doesn't dissolve and migrate up through the new coat. After the sealing coat over bare wood use soapy water/rinse/dry before sanding and applying the next coat of epoxy? I ask because I've read some references that don't mention blush removal at all between coats, or they say to sand off the blush, but that sounds like an inefficient mess and a waste of lots of clogged up sandpaper.

    Also, when I get the boat flipped and cover the hull, is it better to lay the glass transversely, progressively butted up to each run, or longitudinally? I've seen both in videos (the longitudinal approach, for example, was to cover from centerline to the taped off chines, cure, trim, scarf to fair and then overlap to gunwale - took a lot more time as you can imagine).

    Thanks!

    Curtis
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If you get to a situation where you are ready to apply another coat of something, over a previously, but freshly applied coat of epoxy, it's best to assume there's a blush. The only exception to this rule is if you're sure you're in the chemically active window and you're applying more epoxy. In this case you don't need to wash or sand, simply apply more goo.

    Blush is water soluble and just a touch of soap helps, though isn't completely necessary. Any epoxy coating that has cured, should be washed, dried and sanded, so subsequent coats (whatever they may be) can be applied to a clean, toothed surface. Some do say you can sand off blush, but I've found you can spread it around, causing a bigger problem, without the water, so my recommendation to novices and back yard builders is to just assume it's blushed and wash it, if the epoxy is cured.

    If you sandpaper is gumming up, you're trying to sand, yet cured epoxy and you should just wait. No blush doesn't transmute through interlaminate bondings, but it will prevent a good bond if you coat over it.

    It doesn't matter which way you run 'glass cloth, though it does if it's a directional fabric (biax, etc.). I usually do it longitudinally on small craft, but whatever is most practical on larger.

    I have put some new information about painting and stuff on my site in recent days Curtis and thanks for the comments.
     
  7. OrcaSea
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    OrcaSea Senior Member

    Thanks, Paul, for taking the time to respond (and everyone else, as well). It is always very appreciated!

    I scanned some sections late last night, but will definitely re-read your site before mixing any goo.
     
  8. SukiSolo
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    SukiSolo Senior Member

    After washing the bloom, you can be fairly brutal with sand paper grade too. Generally I'll use 80 grit both power and hand. Do the flattest areas first, leave the corners till last and maybe go to 120 grit for those. Leave as much epoxy on the 'edges' as possible, just so easy to go through it!. Also remember to work the diagonals like a fairing board motion even if using a small block.

    If the sand paper clogs a bit, just wire brush it out. Also do not power sand for too long in one spot, it generates enough heat to soften the epoxy and make it 'melt' into clogging balls!. Applies only to new work, fully hardened epoxy should not do this.

    Generally if applying a tape type repair, I'll blend the edges by sanding flush, then apply one more coat of just resin. This helps to seal any 'open' fibre strands that may be exposed from sanding and blending. After cleaning and cutting the cured resin, any coating - varnish, paint etc can be applied. This works well for internal corners, externals may require a little relief prior to applying the cloth/roving if you want a true flush fit after filling.

    Remember to ensure the cloth is fully wetted out - it goes transparent when it is. If you get a 'dry' spot which subsequent sanding may reveal, you can locally blend in (after sanding clean) a small bit of cloth, with a small scarf.
     
  9. michael pierzga
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    michael pierzga Senior Member

    Impossibe to say what the best radius is. We know that sharp corners concentrate loads, we know that sharp corners are subject to chafe, we know that heavy fabric will not conform to a sharp radius. You must take all these factors into consideration when choosing a radius.
    The workman's rule is the bigger the better. 12mm is a very workmanlike radius. If this 12mm radius must be reduced to an 4mm radius for aesthetic or functional reasons it is accomplished with fairing compound after the laminate is completed.
     
  10. michael pierzga
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    michael pierzga Senior Member

    And learn how to use peel ply over the laminate to prevent blush and reduce all the sanding, dust and toxins floating around your shop.
     

  11. OrcaSea
    Joined: Oct 2014
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    OrcaSea Senior Member

    Thanks, everyone for taking the time to respond - lots of great information!

    Michael, my biggest problem regarding radius is that the original builder has a lot of short edge margin screws around the circumference of the deck, so I am limited to how much I can take from the corners. I believe that I can safely do 5-6mm/1/4", but then I am even further reducing the edge margin, so it's a bit of a conundrum for me - leave edge the edge margin that I have, or increase the radius of the edge and reduce the edge margin further?

    The short edge margin raises a troubling fact that the fasteners also have VERY short edge margin into the 3/4" X 2"clamp (or at least what the original drawing refers to it as) that joins the deck and hull (the skin is 3/8" ply). On the other hand, the little boat is probably 40-50 years old and the deck shows no signs of stress or separation, so it has held thus far.

    I suppose I could remove the short edge margin fasteners and move them inboard, but it's a lot of removing and re-placing and hole plugging. Adding a new row of fasteners further inboard to reinforce the existing ones and giving it the biggest radius I can sounds like the best plan. Thoughts?

    Thanks, again!

    Curtis
     
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