Photograph of the keelson of Jonathan McNally's Phoenix III , where you can just see a feint crack in the outer edge of the epoxy fillet on the inside surface of the garboard strake. |
However, Jonathan's story brings up several matters which I'd like to discuss regarding the use of epoxy as an adhesive, and as a surface sealant i.e. epoxy encapsulation. For those who have missed it, here is a link to a recent blog post I put up about the hull structure of Phoenix III after Jonathan first reported his problem.
http://rosslillistonewoodenboat.blogspot.com.au/2016/02/phoenix-iii-hull-structure.html
Two of the possibilities I had considered in regard to Jonathan's cracking problem were: -
- perhaps the planks had been pre-coated with epoxy, and when glued into position on the boat, the cured epoxy coating may not have been adequately sanded; and/or
- the pre-thickened epoxy glue may not have been laid onto a freshly primed gluing surface.
Neither of those possibilities were to blame as it turned out, but they do bring up issues about which people need to be aware.
Epoxy is my favourite marine adhesive by far, although I do make use of a number of other glues for specific jobs. But epoxy is the most versatile adhesive I use, and the vast majority of my gluing is done using epoxy and suitable additives. Epoxy is gap-filling in a truly structural sense, and that is the key to its versatility.
Here you can see how I have primed (or wet-out) two adjoining surfaces prior to laying down thickened epoxy which will be formed into a fillet. |
Now this brings me onto the subject of pre-coating plywood - or any wood for that matter - and what I see as being some stumbling blocks. Pre-coating sheets of plywood laid flat on a bench is certainly convenient, and efficient from the coating application perspective. But the problem is that when the components are cut from those pre-coated sheets, all surfaces which are going to be glued MUST be very well abraded so that the epoxy adhesive (and its priming coat) have a 'key' or 'tooth' to which a mechanical bond can be established. This represents an extra step in the building process, and detracts from gains made through the pre-coating. Also, the mechanical bond between the fresh epoxy and the previously applied pre-coat represents a 'secondary' bond - good if well executed, but not as good as a chemical bond.
I also have concerns about the cured epoxy on pre-coated sheets being subjected to tension and compression when components are bent into position. In my mind's eye, I see micro cracks forming on the tension side of the material, and crushing occurring on the compression side. Unfortunately, I do not have the engineering or chemical qualifications to claim that I know what I'm talking about!
Under some circumstances there may be a place for pre-coating - an example would be the under-surface of a cabin-top or a deck, where subsequent sanding would be very difficult. In that situation, the under-surface could be pre-coated, and then sanded to the point where it is ready to accept adhesive where it sits on deck-beams etc, and it would also be ready to accept paint.
Other than in the cases mentioned in the preceding paragraph, I much prefer to build the boat structurally, and then apply any epoxy coatings. I have fairly strong opinions about where epoxy coating is of value, but that can be the subject of another post.
Thank-you Ross for this interesting post !
ReplyDeleteKind regards
Francois, I am afraid that my comments will upset many people, but it is just an opinion. I have seen many epoxy joints fail because of inexperience and ignorance on the part of the builder. It is a pity to ruin a beautiful boat because of a simple error.
DeleteBest wishes,
Ross
Hi Ross- I have mixed feelings on pre-coating also. On my last major stitch and glue boat, a 16' flat bottom power boat, I did pre-coat the insides of the panels, but masked off the edges where the fillet bonds were to go. This worked very well- her's a link to the photo album on my Facebook page - https://www.facebook.com/media/set/?set=a.671326766251101.1073741842.159470154103434&type=3 . Pre-coating these large panels was too easy to skip, and gave me a much better finish than coating after assembly.
ReplyDeleteHi Ross, thanks for the article. I was was considering using System 3 Silerver Tip Gelmagic Epoxy that doesn't require fillers on my Periwinkle build. They also make Silver Tip Laminating Epoxy which is a medium - low viscosity epoxy that I could prime with. Any thoughts or suggestions?
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteSorry - I hit the wrong button by mistake, and deleted my answer!
ReplyDeleteI believe that Gel Magic is said to be usable without a priming coat, according to the instructions. I would not argue with the chemists who formulate the product, and I hold System 3 in high regard.
However, epoxy is what holds most of our boats together, and these days fastenings are mainly used to hold components together until the glue cures. When I am building I try to remember that the boat mat still be on the water carrying people in forty years........usually I prime my joints! Straight Silvertip would be fine for that - you don't need a lot, and it is simple to apply with a disposable brush or roller.
Learn more about Gominhtien plywood
ReplyDeletehttps://www.skillshare.com/profile/Van-ep-Gominhtien/700869377
https://hub.docker.com/r/gominhtien/van-ep-plywood
https://form.jotform.com/211512367601042
https://www.flickr.com/photos/191194022@N04/51091944564
http://www.wikidot.com/user:info/gominhtien
TOTO's Toto Toto 4/3 TOTO - Titanium Stays
ReplyDeleteTOTO 4/3 babyliss titanium flat iron TOTO. TOTO titanium necklace · TOTO TOTO 2021 ford escape titanium hybrid TOTO 4/3 TOTO. TOTO titanium flashlight · TOTO TOTO TOTO TOTO 4/3 TOTO · TOTO TOTO TOTO 4/3 TOTO. TOTO 4/3 can titanium rings be resized TOTO. TOTO 4/3 TOTO. TOTO. TOTO