Hi rjacobs,

Thought I would try to address your questions as asked.  

On the initial level of sanding.  If you still have a lot of weave showing, I would lightly abraid the surface with a green scotchbrite pad and wipe down with denatured alcohol to remove any dust… and then add a couple more fill coats.  For me, after the initial wet out, I usually have three weave filling coats.   I work carefully so my fill coats are rolled on then I use a plastic squeegee to smooth the surface (knock down any bubbles and move the epoxy into the low spots).  That is usually enough to allow for sanding the surface to a smooth finish without going into the weave.  As for blush, if you are using MAS medium epoxy ..these are non-blushing epoxies and you should not have to worry about blush.

On the structural consideration for micro balloon use…..there are a lot of opinions on this but i limit my use of micro balloons to address final fairing issues (like a depression that needs to be filled).   I don’t substitute it for filling the weave as I agree with the hardness issues you mention in your question.  Also, in my view, you don’t get a surface that is much lighter or easier to sand unless your microballoons are  applied like a paste (e.g., bondo consistency) which is how I use it…..so i want a very good water-proof layer in place before any 'bondo' consisency micro-balloons are applied.

When to do water Trials….i would have all my fill coats in place prior to any water trials.   With only wet out glass, there is still the probability of small micro-openings (e.g. bubbles) that can allow water to get behind the epoxy into the wood.  After a couple of filler coats, that risk is pretty much off the table.  So I agree with your intuition and have seen it in practice that one coat is not a very reliable seal against water intrusion.

hope that was helpful. 

h

Thanks Hspira, yes, that does help.

A couple takeaways:

1. It's probably best for me to stick to at least one more "normal" (unthickened) fill coat on the hull before exploring any micro balloons. Once that's done it'll also be a good milestone for the water test ahead of any faring.
2. I need to get some scotchbrite pads. That's a tool I have yet to make use of.

   Thanks again.

  Seems to me your over thinking things .roll on epoxy until  you don't see the glass weave and take your time sanding.unless your having to cover up mistakes that should  be enough. If you want  you could use a high  build primer on the painted areas but even that  has some drawbacks 

Just a few thoughts on microballoons.

First, concerning structural uses of microballoons. When you see that they are not meant for structural use, that means things like thickeners for glue joints between structural members. Filling the weave is finish, not structure. Once the glass fibers are properly saturated, they are structurally complete. Filling the weave is a cosmetic and protective process, not a structural one. That's why it's perfectly safe, from a structural point of view, to leave the glass in interior areas unfilled.

As far as hardness, microballoon/epoxy mix cures much harder than paint, high-build primers, etc. so in the event of a scratch it will not be what fails first. And scratches are all the you need to be worried about. The glassed wood is what handles impacts, twisting, bending, pulling, etc.

And if you are not sure about taking my word for it, look to the world of home-built, hand-built composite aircraft. Standard practice for scratch builds is to do a glass lay-up with an epoxy content as close as possible to 60% epoxy/40% glass (which will always leave the weave unfilled) and to fill the weave with fairing compound. If that's good enough for a 600 lb, 200 mph  plane that has to sit out on runways, fly through bad weather and endure bumpy landings, it's surely more than enough for a 40 lb boat that speeds along at 7 mph and lives indoors or under a cover. Even cartopping speeds are not a problem.

Filling the weave with unthickened epoxy is a heavy, expensive and unnecessary option unless you are finishing the boat with varnish. It adds no strength beyond what the glass and wood already provide and only marginally more abrasion protection.

Second, fully-cured balloon/epoxy mix is watertight. The balloons are dust speck sized, so even a very thick mix has lots of epoxy in it, plenty to seal micro-cracks. The balloons are small enough to fit into micro-cracks. It'll survive a sea trial just fine.

Finally, I like to do sea trials between the fairing and painting stage. That way I get to test the final shape of the hull but without having to do all the finish sanding.

My 2 cents,

Laszlo

Thanks Laszlo for the additional feedback on the structural considerations of the fill coats. That’s very interesting. As greg27 noted I may be overthinking things here but I really do enjoy learning not just “how” to do this build, but also “why” various steps are done the way they are done (or could be done differently).

For now I’m building up 2 fill coats on the hull before switching to micro balloons, but in a future build I think I’ll try making the transition earlier. Minimizing weight will be a fun new challenge for a future build, even if it's just a few ounces here and there.