How to glass a window rebate into a panel?

Discussion in 'Fiberglass and Composite Boat Building' started by Midday Gun, Feb 11, 2022.

  1. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    If you wanted to glass a rebate into an (already existing panel) to allow for a flush fitting window, how would you go about it?
    I have some ideas, but they aren't fully formed yet.

    Existing setup has aluminum frames & windows, they're forever leaking & the aluminum is starting to show signs of corrosion.
    I thought recessed windows might be an interesting project to have a go at.

    upload_2022-2-11_21-5-42.png

    Above is the current setup, so single core in coach roof sides.
    Layup as is:
    upload_2022-2-11_21-6-27.png

    5mm acrylic + extra for the thickness of sealant required to bed it will be thicker than the existing layup.

    The idea I had was to make a blank up that was the same thickness as the acrylic + sealant will be, chamfer the edges, mount it flush in the windows, tons of release wax on the back of it & then glass over it from inside the boat.
    Finally use thickened resin on the outside to smooth out inside the radius o the join before applying gelcoat & cutting the hole for the window in the rebate.

    Does that all seem sensible?
    I'd thought maybe I should add some core around the window to stiffen & thicken that area up a bit, but there's some fore & aft camber on the coach roof side anyway which makes for a stiffer panel anyway.
     
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  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    You want to have a flush window set in adhesive with no fasteners? If the frame section is exactly how you drew it, that is why there are leaks. There are two flat surfaces which does not allow enough caulk to seal properly. There should be a groove in the frame, cabin side or both to allow for the caulk to form an "o-ring" and allow for movement.
     
  3. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    These are my two ideas:
    upload_2022-2-11_21-38-37.jpeg

    In the cored example, my idea would be to cut the core perpendicular to the panel, and seal it using thickened resin to form a chamfer. With the window installed the gap between the edge of the acrylic and the chamfer would be filled in with sealant.
     
  4. Midday Gun
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    Midday Gun Junior Member

    Hi Gonzo, the drawing is not fully representative of the actual installation on the boat, its just how the designer penciled it in back in the eighties.
    The windows have perspex sat in an aluminum channel. They are in two pieces that slide on from each side & have a butt strap joining them. The whole assembly is then bolted onto the coach roof (with sealant as well). The frames are starting to look tired though & the cost of new is extortionate.

    I would like to go to as you described, bonded in acrylic without fasteners.
     
  5. wet feet
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    wet feet Senior Member

    A couple of points; its normally considered good practice to balance the laminate either side of the core and second,I'd choose good foam over balsa every time because it won't rot and balsa does in many cases.
     
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  6. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    I had planned to use foam if I did core it, I'm not entirely sure if its necessary though.
    The existing laminate is pretty basic, around the window areas, no core, so just 4 layers of 600gsm, 1 layer of 300 & the gelcoat.

    To match that same thickness on the inside of a core when its been adequate for 30 years seems to be a lot weight & resin I don't need to do?
     
  7. wet feet
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    wet feet Senior Member

    Do keep in mind that the metal frames will have been adding stiffness to the area and it would only take an additional 300 g/sq.m after the gel to give a balanced laminate.
     
  8. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    I'm a touch confused. By balanced, I thought you meant that I should have even layers on each side of the core?
    My initial thoughts had been:
    - Existing laminate
    - Core
    - 300gsm matt.
    - 2 layers of 600 biax.

    Then in the area where it transitions to single skin for the rebate, some extra layers of glass to build up the thickness.
    However I'm not sure if that's the best idea as around the window it will be fully relying on the peel adhesion of the laminate to the core & the core to the old laminate.

    Its probably better to grind back outside & glass the rebate from the outside to the inside to get better strength?
     
  9. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    I suspect, hard to say without more evidence, that it is an issue of stiffness.
    You appear to have a large distance between the structural support and the beginning of the window:

    upload_2022-2-12_8-50-38.png

    The distance creates a bending moment, once the window has any load on it, as well as general hull flexing.
    This region of the laminate is much less stiff than the aluminium window frame. So I suspect this region is flexing and as such, allows a path for water to get in and under the flange of the aluminium frame.
    If you can reduce this span/distance and/or increase its stiffness, it may well help.

    If you're getting corrosion, you have dissimilar metals and thus and electrical path for the electrolyte to cause havoc.
    So what is located nearby, or what else is touching/attached to the aluminium frame and what material is it?
     
  10. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    Stainless fasteners, aluminum window frame is the corrosion source,. These things were fitted to thousands of boats in the UK, they all leak eventually, the leaks aren't from the frames to the GRP coach roof, they're in the acrylic to frame bonding.

    But this is all outside the scope of my question, I'm going with frameless acrylic, the original plan was to just bond them on the outside of the existing skin as many boats already do, but I've got a fair bit of laminating work I'm doing elsewhere, so I thought it might be an interesting project to have a go at making them flush fit. I'm not decided on if I will do it or not, but I'm just trying to decide the best way to do it, if I do.

    If it helps, this is the existing arrangement. (Excuse the dirty decks, this was just after I bought her, years ago)
    upload_2022-2-12_0-46-9.png
     
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  11. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Do you want to keep your windows the same size and shape as they are presently on your MG?

    If not, would it be feasible to glass in the existing aperture and then cut a new aperture for something like an 'off the shelf' Lewmar portlight or similar?
    Standard Portlight Fixed - White Trim | Lewmar https://www.lewmar.com/node/11226

    If you want to fit a flush acrylic window, would it be possible to build a fibreglass ring bonded to the inside of the cabin that is the same shape as the existing window, but the aperture is smaller to allow a decent overlap for a flush acrylic window to be bedded down on it with 5200 or similar adhesive?
     
  12. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    I had planned to make some small modifications to the windows shape, but would rather avoid an off the shelf portlight.
    Also I believe that the curvature of the side of my cabin is outside the allowance of those Lewmar frames.

    I relocated the chain plates aft & outboard a couple of years back. (I paid a naval architect to do the calcs & worked from their drawings and schedule) But they now fall across the middle of the window, and I had an idea of having two separate port lights with a the area between them providing a much better pathway for the lower shrouds that want to squeeze the sides of the hull together.

    Your idea is one that could maybe work as well, I think it would need to be rebated as the existing laminate isn't thick enough for the window + sealant to sit flush.
     
  13. wet feet
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    wet feet Senior Member

    The useful suggestions above ought to lead to a good outcome.Grinding a little way into the existing laminate will give a better bond than either a solvent wipe or a light sanding and increases the bonding area by a useful amount and it permits a degree of tapering.The blank that forms the recess will need to be a touch larger and thicker than the replacement window,to allow space for the sealant and in the UK you may well find Sikaflex a bit more easily than 5200.Does the plan include the use of fasteners,or is it intended to rely solely on the adhesive properties of the sealant?If the latter,it can be challenging to apply the force to hold everything in place while the sealant cures.
     
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  14. gonzo
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    gonzo Senior Member

    You should get the proper caulk/adhesive for the material you are bonding. Polycarbonate and acrylic use different products. I find that Sika has the better options.
     
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  15. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    I would avoid building the flange in place since it would mean a lot of grinding and fairing inside the boat. The place you buy the acrylic from probably also sells premade GRP plate, maybe they even offer a cutting service. Buy 12mm plate, cut a frame about 50-60mm wide, round over the inside edges and cut the needed rebate (don't forget to add the mandatory glue thickness to the acrylic), paint the inside face on the bench. Stick to the boat with thickened epoxy using short (do not go trough the flange) screws trough the existing holes. Clean the squeezeout completely and tidy, remove screws and fill the holes with polyester structural putty, then gelcoat over with a small brush.
    You can of course build your own flanges on the bench, solid or cored instead of using premade plate.
    When grinding the inside of the cabin side, do it from the outside trough the porthole. Tape a big plastic sheet on the inside so that it is loose enough to allow you to do the work, then vacuum it, spray with water and remove.
     
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