View Full Version : Epoxying
Erik le Rouge
09-15-2006, 07:37 AM
Would it be a good idea to saturate the planks of a plywood planked boat or S & G boat prior to assemble it ?
09-15-2006, 07:53 AM
You want the wood to be in its final bent and twisted form before adding epoxy.
Most epoxies (except CPES) don't really saturate more than a cell or two in along the grain but they do travel well in an exposed end grain. This will give you hard spots in the plywood.
With plywood, it is well to saturate all edges as the plank goes on to be sure the grain has accepted all the epoxy it can and won't leave a dry spot in any glued joints between planks or between plank and stem or whatever.
09-15-2006, 08:07 AM
Many professional boatbuilders, like John Brooks, say not to use epoxy as a coating at all on plywood boats.
09-15-2006, 08:28 AM
I built a Bolger ply dingy ,I left a few parts uncoated without epoxy,the rest was epoxy coated.
One year later and rot was happening in the uncoated area,coat everything in a ply boat!
This is a photo of a preglassed sheet of plywood 18' x 4' I made my latest kayak from. Some say this is a bad idea because the glass/epoxy must then be stretched along the outside of the curved panels and compressed along the inside curve when the boat is assembled. I think it depends. On the shallow bends 3 mil panels of a kayak make it doesn't seem to matter. People who disagree with me say it does indeed matter even though one cannot easily see the harm I've done and I'll be sorry down the road when my boat falls apart. I've made four boats this way and none have fallen apart yet. I have found through experimentation that it's not such a good idea on tighter bends and that the glass/epoxy stretches slightly over the outside of a bend better than it compresses on the inside.
09-15-2006, 08:59 AM
On the question of when to do the coating (if indeed you are going to coat the plywood) a key question is how much twist and curve the planks need to take on when they are put on the boat. If serious twisting or bending is called for then you definitely do not want to apply the epoxy first, because it will make the plywood stiffer. On the other hand, if the twists and bends are relatively mild then a good argument can be made for "flowcoating" the plywood before you even start to cut up the sheet of plywood into planks. "Flowcoating" is a method of effectively applying 3 coats of epoxy in one go and very quickly achieving a surface that can be sanded smooth with little effort. Coating this way is MUCH quicker than coating after the fact, when you have to apply the epoxy one coat at a time and on top of that do more sanding in places that are harder to sand. Yes, you waste a little epoxy because you are coating some wood that will not be used, but the labor savings is huge. Just make sure to completely coat the edges at some point in the process after you have cut out the planks. If you are going to be cutting laps I would be more hesitant about flowcoating because it will make cutting laps a lot more work.
On whether to coat at all, I am not going to delve too far into this argument. I will just say that a boat that is rotting one year later sounds like a boat that was not kept well painted and was stored in a very exposed location. I am not saying Jim did something wrong -- sometimes the use of a boat makes it hard to keep up with keeping paint on places where it is getting scraped off, and sometimes boats have to be stored in exposed and "hazardous" locations like dinghy docks. On the other hand, if your boat is going to be used carefully and stored in a garage then rot should be much less of an issue. Of course down the road the storage location as well as the care with which the boat is maintained may well change.
Flowcoating is covered in the Gougeon Brothers book, or, if you are not familiar with it and don't have the book, say so, and I or someone else will try to come up with a description.
If the plans say to coat everything I would be inclined to follow the plans, since the designer may have factored in the stiffening effect of the epoxy coating.
Following Bruce's post, I really don't find flowcoating or even preglassing as I do stiffens the panels very appreciably. It may even help bending because the slightly stretchy sheath acts as a backing to the outside of the panel.
09-15-2006, 11:01 AM
There it is. Different boats, different longsplices.
Erik le Rouge
09-15-2006, 06:15 PM
The boat I want to precoat has soft curves as it is a kind of sharpie skiff. So I thank you for your information.
By the way I like that forum a lot : The dedication of all of you and the different approaches to boatbuilding that finally point to one way, passion in our hobbie.
09-15-2006, 06:29 PM
get a 4 x 8 sheet of formica, glue it down to something solid....
wax liberally and smoothly with Johnsons paste wax....paint with Polyvinylacetate..(blue looking syrupy stuff...) let it set for a couple of hours.....then paint your ply with low viscosity epoxy......then....either put on a full strength heavy coat of epoxy/alternately...lay on a layer of xynole fabric and epoxy.....turn face down one the formica.....place some cinder blocks on top for weight.....next day remove blocks and plywood and you have a slick formica smooth surface......
On stitch and glue hulls, in many designs...it makes sense to glass panels before stitching together...and there have been some glued ply lap designs where the planks were glassed before bonding in place. Glassing offers a tremendous amount of protection and a great barrier to moisture.
In general, I would think just epoxy coating could be done just as well after assembly unless large panels that will not be bent much... and it is much easier to epoxy coat them on the horizontal before assembly...
Lots of this is pretty much common sense... if you consider how easy or difficult it will be to epoxy coat a surface after assembly...
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