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OB-20, Epoxy Pre-coating of plywood

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Hi All, nearly ready to start my OB-20 and I was just wondering if there is any merit in pre-coating the plywood panels with epoxy? I envisage it may save a lot of hassles when it comes to getting into the smallish areas inside and also to make sure you get good coverage for sealing. Would this be an advantage or not?

Trev

 

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For some areas, it is a good idea. For others, not so much. On the Princess I'm building, I intend to pre-coat as many panels as possible.

 

Gravity works and you are far and away better off coating and sealing large flat areas while the panel can be laid flat and level. You won't get the runs and drips you would get if the panel is vertical or tilted. Runs and drips will give you fits trying to sand them level later on, so you want to avoid them if you can. Use a low viscosity, non-blushing epoxy and coat it hot on hot (roll on a coat, wait until it goes tacky, then roll on another coat, etc). Hot on hot gives you a primary bond on all your coatings. Three evenly applied coats should enough to seal most areas that will not be glassed, such as your interior panels. Low viscosity (runny) epoxy will do a better job of self leveling and most of the bubbles in your mix will float to the top and pop, thus avoiding the pin holes later on. You can also pre-sand those areas so they are ready to finish once the panel is installed.  But all that advice applies to the large open areas of flat panels.

 

Where it is a bad idea is along the margin of those panels where you may be using tape and fillets to join two panels together. In those areas, you want a primary bond to each piece. If you precoated the edges, you would either have to sand them back to bare wood or be left with a weaker, secondary bond. Given that choice, I'd always go with the stronger, primary bond and deal with the flat panel area later.

 

If you have a good idea where your taped edges will be (this also applies to any cleats, clamps or carlins that run through the middle of the panel), you can always mask those off with tape and coat the main field of the panel. A small area that is mostly hidden from view, with several taped edges, etc. is probably best assembled without pre-coating. Items like locker panels, bulkheads, etc. can be precoated, provided you do leave that uncoated margin along the edges where fillets and tape will be used. Any panels that have to curve as they are installed are best left to coat later on, as the curves subject the panel surfaces to tension and compression, and any coating you put on beforehand will interfer with that, not to mention put the coating under the same forces, which may cause them to crack. An item like a berth panel that gets installed flat could be pre-coated on the bottom, then final coated on top after the panel is installed, provided it is flat and level.

 

Just keep in mind gravity works and it is always easier to get a good finish when the panel is flat. But easy always takes a back seat to best when the joint is structural and holds the boat together.

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Hi Trev,

 

I have now built 6 boats over the years and the first 4 were pre coated full sheets of ply then sanded. The last 2 I listened to the designers and epoxy coated after construction. I will probably pre coat my next boat. It is much easier to coat the flat sheets of ply, you don't get any runs and it is much easier to sand a large flat sheet of plywood.

People will talk about primary and secondary bonding. I have had no problems in the past and don't expect to in the future using both methods. Do a test with your ply and the epoxy that you are going to use and see for yourself. I have seen many boats coated after construction has completed and they were left with a heap of runs to then go and sand off.

 

Alex.

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I agree with Alex, I to have built several boats over the years, and I have found pre coating the panels much easier than after assembly. If your coatings are done properly, I doubt that your fillets and glass would be an issue, if they are sanded primer to coating. Check with your epoxy supplier, I would suggest they will agree. I use mas epoxy with no issues.

Greg

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Thanks Greg, I was also wondering how the 4mm will bend, after having 3 coats of resin? In the front section of the sides, it (the side panels) have 2 layers to get the flare in this area?

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Pre-coating tends to be application specific. Some panels will be tough to force into shape, so if precoated, will just fight you some more. Other panels might need a fillet or glue line on several areas and the best bond would be against raw wood, not precoated. Of course, you could tape off these area, but this requires more forethought and planing then most home builders employ. Lastly precoating usually means you'll spread epoxy onto areas that will eventually be cut away as waste.

 

So, yep, it can speed things up, but it can also waste goo, require more effort (taping off seams and bond areas, etc.), not to mention require prepping of precoated areas for a mechanical bond, where you might prefer a chemical or raw wood bond. In short, if you think about it, you'll find places where it will save you trouble and others where it'll cause you difficulty, so sit down, think about it and precoat where it'll be of most benefit.

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I am convinced precoating is the way to go. One trick I tried is noodling out areas that would be receiving glued in panels and/or taped joints and then while the precoats were still wet I ran a strip of 2" wide peel ply tape on to-be-glued/taped areas and squeegeed it down. This negates any amine blush problem and leaves enough "tooth" for epoxy to grab onto after you peel the peel ply off. Two boats that way, zero problems.

If you are bending 4mm into a fairly severe curve while stitching it up, a well cured epoxy joint will fight you, especially if the coated side is the outside curve. Solution: Plan to coat the wood, then stitch it up while the epoxy is still green and quite flexible.

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I have been reading this discussion rather thoroughly and trying not to formulate opinions that are not backed by evidence or at least are the opinions of experts with some what extensive experience.  One professional has spoken of the importance of not pre-coating areas where structural adhesive joints will be made.  One amateur has mentioned that the designer of his projects recommends against pre-coating where adhesive joints will be secured.  In spite of this several amateurs are saying they will pre-coat everything anyway and some are even recommending pre-coating everything anyway.  The only evidence given by any of the amateurs is that they, with rather limited experience, have not run into problems yet. (key word here being "YET")  Sorry, but I just don't understand this.

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I might as well add a bit of confusion to the pre-coating spiel.

 

Precoating after the panels have been cut out avoids wasting lots of epoxy and sanding on areas that will not be part of the boat.  After scarfing the four bottom and side panels and marking out areas along the chine, keel, stem and transom, precoat all the rest laying flat. Same for any bulkheads after they are cut and trial fitted.  Most of the plywood is now in near finished form ready for fold up.  Locations of bulkheads can be ground clear of epoxy after the boat is formed.  If you are smart enough to find the locations of bulkheads, these areas can be left clear but that may not be as simple and you might think.

 

On a kit boat all this is somewhat simpler since the areas that will be fileted and taped are at least partially marked by assembly tabs.

 

Having built many boats and given all this advice, I admit that I have always formed up S&G boats before laying on the epoxy.  Still, I see the advantage of coating first and admire the neatness of others.

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