http://www.morocz.com/BoatBuilding/DuckBuild9.htm. Not sure if this is what you are looking for but it does have graphite and epoxy on the bt. of a wood duck.

Tom

It's not so much the ratio you need to be concerned with (volume? weight?) it's the consistency of the mixture you create then spread out onto your weave.

By volume it can be 1:20 or more by volume, epoxy (resin+hardener) to microballoons, maybe even more. I work in enough MB to come up with a mixture about what I remember from when I used BONDO decades ago, or a similar but less known productb called TUFF-CARVE.

It needs to be thick enough to avoid 'slumping' or running off vertical surfaces but not so thick that it becomes hard to spread evenly with a plastic squeegee or closed-cell packing foam spreader. I stop adding MB to the epoxy mixture when it shows signs of being stiff enough not to 'self-level' after stirring. 

It's not a structural mix either by any stretch! The epoxy simply serves as the binder, the MB serves as the filler that does the actual job. Too, the higher the proportion of epoxy to MB the harder the resulting layer will be, the greater its resistance to easy sanding.

And you MUST add at least one more coat of unthickened epoxy (with or withought fiberglass cloth) once you've faired your surface after the filler mixture's 100% cured. All those 'balloons' you sanded open need to be filled with epoxy before any further finishing work is done, unless it's adding another layer of MB fairing mixture.

Using graphite as a filler the ratio can be more variable. I'd work a mix that's easy to stir (once the resin+hardener have been thoroughly mixed) by adding powdered graphite until it's the consistency of mustard. Easy enough to roller on or spread with the plastic squeegee or packing foam spreader. It's a finishing mixture, not a weave-filler in this instance.    

For graphite epoxy I now use 50:50 by volume since I'm mostly paddling in areas with soft muddy bottoms with the occasional oyster shell. For harder bottoms use more epoxy and less graphite.

For the microballoons, it's as spclark says, you go by consistency. For just filling the weave I make it a bit runny, like French's mustard right out of the fridge, and don't use much. For fairing and when I want to fill and fair with just one coat I make it about the thickness of canned cake frosting at room temperature.

And respectfully disagreeing with spclark, I've never coated the final sanded epoxy/microballoons with anything but paint. A good quality marine paint (I prefer 2-part linear polyurethanes with a cross-linker for the final coat) will easily fill the broken balloons. Those things are small, like a fine dust, and the holes are invisible to the naked eye. The surface is as smooth as a well-sanded primer and the resulting finish can be yacht-quality. I've never had any problems with water absorption, peeling, etc. and some of the finishes are 14 years old and still going strong.

Use a longboard to hand sand it, it will give you the best results.

Laszlo

   Any tips for keeping the transition from graphite to the bright portion of the hull clean and even?  I dot plan to sand the graphite so how do I blend them together nicely or do you just accept they will have an offset?  On a separate note any concerns with heat and humidity when applying epoxy beyond quicker setting time?  Living in southern AZ during monsoon. 

I use the fine line masking tape sold by CLC to keep the edge clean and even. I never sand the graphite/epoxy. It's way too messy (nasty fine black dust everywhere) and doesn't really accomplish much of anything for me. I'm OK with a glossy black surface that is not perfectly smooth vs. a smooth dull gray surface and looking like a 1700's coal miner tracking black footprints into the house. The tiny ridge where the graphite/epoxy and varnish meet doesn't bother me, either. It's invisible and can only be detected by touch.

If you've never sanded graphite/epoxy before, it's astonishing how dirty the process is. I would recommend that you make up a test piece of wood and try it on that before you commit yourself to doing your whole boat.

Laszlo

   Re: graphite powder

I actually read the can this time before adding a bead on a JS 2 skeg and it pointed out that besides being nasty stuff to work with the powder in its virgin state is electrically conductive. Hopefully thoroughly mixing it with epoxy will encacilate it so my orbital sander will survive.

You'll be happier wetsanding it by hand.  

   Sorry if there was confusion but no plans to sand the graphite section.  Long as the seem between bright and graphite looks decent I want to keep it glossy black. 

   Sorry if there was confusion but no plans to sand the graphite section.  Long as the seem between bright and graphite looks decent I want to keep it glossy black.