Marine Fir plywood edge adhesion

Discussion in 'Materials' started by Metalbuffer1, Jul 6, 2021.

  1. Metalbuffer1
    Joined: Jul 2021
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    Metalbuffer1 Junior Member

    Hello everyone, my first time here. Even thought I grew up 10 feet from the ocean my question is not about boats directly. I’m going to rip 3/4” marine Canadian fir plywood in to 4” strips 4’ long. Stack them with epoxy to bond them together. This piece will get several coatings. Starting with epoxy, then vinylester, then polyester then hi end nano clear coat. I work with all these materials all the time but never on a surface of All edges. My main concern is the epoxy to the edges. Not going to get any penetration from the epoxy into the surface, so do you have any advice for the initial coating to get best bite into this full edge surface. This piece will be exposed to the elements but not for a below water usage. Thank you.
     
  2. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Welcome to the group.

    All that edges will open, split and desalination unless reinforced.

    Use fiberglass cloth in the base layer of epoxy. 6oz woven should be invisible. Be sure that the clear coat provides sufficient UV protection to the epoxy.

    Why the multiple layers?
    Most clear coats can be applied directly to epoxy.
     
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  3. Metalbuffer1
    Joined: Jul 2021
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    Metalbuffer1 Junior Member

    I’m using multiple layers because my last resin layer has to be polyester. This is a polyester product we use with metal fillers for causemetic reasons. I want to use epoxy for first layer for bond, then ve as a go between the poly and epoxy. Mainly this build up will protect the epoxy from the sun and I hoped the layers would keep me from using any fiber glass. Also the surface itself will be scored for textural reasons before any coatings are applied. But if the edges of the plywood will delaminate under all of this I guess I need to rethink it.
     
  4. Metalbuffer1
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    Metalbuffer1 Junior Member

    Just a photo I pulled off the web. But this structure with several protective coatings I think should be ok. I understand it will be outside but I wouldn’t think it would come apart. But this is why I’m looking for advice.
     

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  5. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    You are fighting two enemies- water penetration and thermal expansion/contraction.

    You have the first conquered.

    Only FG reinforcement will defeat the second.
     
  6. Metalbuffer1
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    Metalbuffer1 Junior Member

    Blueknarr would delamination still be an issue if this was laminated with a sheet say 1/4” marine fir. Or would it still need Fiberglass. Thanks for your help.
     
  7. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    BK is correct. Plywood will delaminate due to variation in thermal expansion rates between glues and wood grains going different ways.

    Even a light fiberglass like 6 oz will struggle under a bright finish if the wood colors vary.

    The 6mm ply will reduce the problem, but will face check and edge delam itself as well without fiberglass.

    Keep in mind, fiberglass is invisible in any woven under 6oz unless you fail to fill the weave with epoxy.
     
  8. Metalbuffer1
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    Metalbuffer1 Junior Member

    I totally get it. Just wanted to clear one thing up, I’m not looking to have the wood showing at all. The polyester coating will be filled with metal and everything under it will not be visible. So having said that it doesn’t matter about Fiberglass being visible, how much Fiberglass is enough to keep it from delaminating? Thanks guys.
     
  9. philSweet
    Joined: May 2008
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    philSweet Senior Member

    This is a pretty common way to use up scrap. How many layers are you going to stack? Can you endplate the stack with a decent thickness of solid wood (like they did in the image you posted)? For ease of assembly, drill the stack and clamp with threaded rod, then apply endplates. This lets you gang sand the assembly before getting any goo out. For best results, use 1/2 ounce mat between the ply layers to ensure adequate resin wetout.

    Make sure you start with a warm-soaked assembly and let it cool as the epoxy sets so you avoid outgassing into the epoxy. Try to keep the edges clean of epoxy when clamping the stack (keep wiping them clean). You don't want to sand all that epoxy off when you gang sand the assembly one last time. Glue endplates the same way.

    Knock off glue globs with a chisel and gang sand to final shape. Fill any voids with an epoxy filler. Heat soak the assembly to about 20 degrees F warmer than the air. Mix epoxy at room temp and then transfer to a cool Teflon frypan (not so cool to cause condensation, just cool enough to stop exotherm from being a problem for ten minutes. Coat the assembly as it slowly cools. You want a decently fast setting epoxy for this - tack free in an hour or two. You will need at least three coats before outgassing is no longer an issue. Either get all of them on in one go over four hours in cooling weather, or do one coat each evening using a slow-to-cure resin (one that has some fast hardener and some very slow hardener to promote intercoat bonding for more than 24 hours).

    This is a very time consuming way to build something. There's lots and lots of sanding. Use the best saw blade you have to rip the planks. But it will be decently stable if you start with good materials. Fir is harder to work this way than okume or meranti. Thick plies are also more difficult, and fir tends to have fewer plies than hardwood sheets.
     
  10. Metalbuffer1
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    Metalbuffer1 Junior Member

    Thanks for this @philSweet. There is a lot for me to learn here in this answer but it all makes sense. I’m going to be doing this to build a door. Door will be give or take an inch, 4’ x8’. 3”-4” thick. The strips will also have the interiors cut out. So front and back will maybe be an inch thick, sides will be about 4” thick. The center will be poured in with expanding foam for insulation. Yes I plan to do all edges with thick solid hard wood. It will be a pivot door. It’s going to get heavy. On thing you said I find especially interesting is putting Fiberglass and epoxy between each ply. As for sanding I’m ok with that. Most of my time is spent sanding. For some reason I enjoy it. It will get tricky putting fg between each ply. I’m going to need to read your post again to get everything out of it. Do you mind if I come back and ask a question or two. I really appreciate this. Thank you.
     
  11. fallguy
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    fallguy Senior Member

    Sun is the main enemy.

    Most of the builders I have read suggest minimum of 2 layers of 6 oz glass over exposed endgrains..I would never leave it unglassed and have only read guys giving failure stories.

    color is another story...anything dark colored suffers more
     
  12. fallguy
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    fallguy Senior Member

    Adding a glass layer ensures even bond joint thickness and avoids dryjoint squeezeout.. Ideal glue joint thickness is 40 mils or 1 millimeter, if I recall my math.

    It is easy to squeeze out too much when mechanically clamping.
     
  13. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Questions:
    1. What is the reason for cutting strips and stacking them?
    2. Why expanding foam?
    2. Did you do an adhesion test of your polyester coating over epoxy? Have you asked the manufacturer for a suitable primer?

    Unless you are reusing scrap, it makes no sense to stack strips. Simply glue the sheets to the desired thickness and glue them to the frame. Even better, buy plywood of the desired thickness, or use some local stiffeners.
    Fill the frame cutout with rigid foam, it's simpler.
    Fir plywood faces will check with time, so if you want the finish to last the ply must be fiberglassed. On endgrain the best strategy would be to use a thickened epoxy without fiberglass.
    Polyester products can bond to cured epoxy, and if yours is specifically designed as a surface finish like a paint, there are often primers to ensure adhesion.
     
  14. Metalbuffer1
    Joined: Jul 2021
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    Metalbuffer1 Junior Member

    Hi Rumars. I get your concern and fully understand your points. I’m very well versed in all these things. Resins of all sorts and primers are my life. Ridged EPS or extruded styrofoam are great material I use them all the time however poured foam is far more ridged in higher densities. Not to mention if poured into a structure it sticks and becomes a magnificent reinforcement because it is adhered through out. Ridged foam would be just sitting in there. Yes it can be glued but will never offer much adhesion. Poured foam in hi density has greater R- Value. Two sheets of plywood glued together will warp. I’m not saying it’s going to curl up like a sheet of paper but from corner to corner diagonally I bet a warm Guinness it won’t be flat every day. I know I’m selling it short because by the time use a frame and stiffeners it’s going to be very very flat every day. I do this also when building a door. The stacking comes in mainly for one reason, freedom of shape. From door to door I have no set up in changing anything. No jig, clamping set ups nothing. Changing the door surface to have elevation changes or going to a curved door horizontal or curved to vertical is as easy as just cutting the strips that way. No frame to change no set up at all. The hollowed out rings of plywood get help from its shape to keep that way. Breaking down any type of wood release it’s inherited flaws,even plywood. After all that’s what plywood is. So stacking gives me freedom that’s all. My dilemma is all those ends all together, this is where my knowledge stops and suspicion sets in. So I’m here knowing there is good advise, and so far it’s been great advise. Side note; when Fiberglass doors and windows first started hitting the market it seemed like it was the answer to slot of problem in the industry. However doors and windows were being recalled like crazy from warping, really bad warping. It all got figured out in the long run from a lot of discussion and engineering. So I just don’t want something unforeseen coming up later especially when all I had todo was ask. I have a question for you, what are your thoughts on Baltic Birch for this instead of fir. Thanks Rumars
     

  15. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    I've worked with composite door manufacturers, it didn't take very long to get it right, and they knew how to do it correctly even before production was started. But cutting costs on spreadsheets creates problems when bean counters determine the production materials and processes.

    Epoxy and glass will create a solid and long lasting door.

    You won't really need any VE resin, use a VE primer, it will make fairing the epoxy and glass surface much easier.

    The exact location and orientation of the door makes a big difference too. If the door faces more of a northerly direction, and has a roof or covered area in front of it, it isn't hard to make last a very long time.

    Facing it south with no covered area means it will see drastic thermal shock conditions on a daily basis.

    In Alberta during the winter, a black door may see temperature swings from -20F at night, to 150F during the day. Plus have 70F on the inside on a continuous basis.

    I spent time in Alberta collecting data on thermal shock causing failures in composite products when I worked for a resin and gel coat company.

    Products made in the US, then shipped to Alberta had a tendency to crack in as soon as a month or so during the winter. This didn't happen anywhere else as often in North America compared to Alberta.

    Part of this was due to the harsh conditions, but also due to the huge influx of money and wealth from oil and other things. This money brought all the boats, RVs and other toys to the region in large numbers.

    Large flat surface with a core tended to crack more often than curved surfaces with no core.

    The core would increase the temperature swing of the outer surface, and increase the temperature differential between the inner and outer surfaces.
     
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