A Photo & Discussion Forum for Wharram Design Enthusiasts
I am a total newbie to boatbuilding and I am constructing a Melanesia.
I have my panels cut out from the plywood sheets and I am about to glue the buttstraps, stringers, laminate the stem and stern posts. To do that, am I going to use the Epoxy resin + filler just like for the fillets?
Do I have to coat the surfaces to be glued previously? (In the booklet it says: "Endgrain has to be coated with pure resin first, as it sucks the resin out of the glue mixture..." I am not totally sure what endgrain means)
I am finding it difficult to find in my country microfibres, colloidal silica or microspheres... A surfboard shaper told me I could use fine sawdust as a filler... would you recommend that? Could I use it both for glueing the parts and for the fillets?
Hi - short answer, no you don't use the fillet mix for gluing! I would suggest you check out some of the "how to" guides that are freely available online for example http://www.westsystem.com/ss/use-guides/ , there you will also find the link to download the "Gougeon brothers on boat construction" which is the classic (I think updated) text on the subject of using wood and epoxy for boat construction.
Endgrain is the part of the wood exposed when you cut across the grain - it is easy to see that it is more porous looking. The method is to coat these parts with pure epoxy (ie epoxy + hardner), then the glue mix has something to attach to. You do this also for the plywood surfaces to be joined. For gluing I usually use microfibres 403, and for fillets the "filleting blend" 405B (West system numbers). You can use additives like sawdust, but I don't have experience of using it for a whole project. Basically the additives give the epoxy something to attach to, to give the mass without being too brittle and allowing you to make fillets which would be impossible if it was just a runny liquid.
Good luck with your build!
Sawdust is all but worthless as an epoxy filler in boat building. It adds nothing to the strength and is hard to sand. Its only possible use is as a coloring agent for cosmetics purposes.
Most wood to wood joints will benefit from the application of a straight epoxy mixture to "pre-wet" the wood before the thickened epoxy is introduced into the joint. Anywhere from 15 minutes to a couple of hours may be needed to provide sufficient time for the epoxy to soak into the wood.
Fillers are used to control and modify the consistency and final strength of the epoxy. They are always added, after the resin and hardener have been thoroughly mixed, in accordance with the manufacturers directions. The addition of cotton fibers help in wood to wood joints. Fumed silica (colloidal silica, Cabosil or Aerosil) helps control the amount of sag in the epoxy, but is extremely hard to sand. Other fillers, like micro-bubbles and talc, are used to make filleting and fairing compounds.
Combinations of additives may be premixed and stored for usage. I use a combination of 1 Part Cotton Fibers, 2 Parts Fumed Silica, and 3 Parts Micro Bubbles as a filleting compound. I use 1 Part Fumed Silica and 4 Parts Micro Bubbles as a fairing compound. These mixtures are based upon the work of others and I've found them to be very effective when properly mixed and applied.
Endgrain : in a ply sheet the width of the sheet (as opposed to the faces). In a length of wood the ends.
You can glue with resin and hardener, if the surface to glue is vertical and the epoxy mix will not stay put or if there are voids between the surfaces to be glued you can thicken the epoxy mix with sawdust or microfibers, even with silica. But don't overdo it, just thick enough.
Between coats of epoxy or before glueing something which is coated you must sand the surfaces, or you can apply the next coat (or glue) before the previous coat has cured (when it's still sticky but not wet, couple of hours) which will make an even better bond as there will be a chemical bond (as opposed to a mechanical bond when sanded) between the coats.
For fillets Wharram recommends : "Low density epoxy fillets are made of epoxy resin and hardener to which are added Microspheres (white) or Microballoons (brown) and some Colloidal Silica. The proportion of the fillers should be approximately 90% spheres/balloons to 10% Silica. However a higher percentage of Colloidal Silica is not detrimental and makes the fillets harder and stronger. You can make a dry blend of the two fillers in a large container, so that it can be directly added to the epoxy, but make sure that they are thoroughly mixed."
Thank you all for your advice!
As you say, Ricardo, I think the best will be to glue just applying a coat that is not left to cure... just like the videos on West Systems show (http://www.westsystem.com/ss/basic-bonding-techniques/)
I will forget about sawdust and go for the proper stuff, as you say Omar.
Éric: nice to see that someone shares my confusion. Only a frenchman could have such a fine appreciation of the gastronomic qualities of the epoxy! I will tell you myself once I try the mayonnaise and the peanut butter.