I'll have to try the plastic overlay approach sometime... it sounds
interesting.

Brian


"derbyrm" wrote in message
m...
I screwed up on my first boat with the wide over narrow, wet on wet. Now I
use the plastic overlay and avoid almost all the effort you describe while
still getting a high glass/epoxy ratio for taped joins. It doesn't really
matter which goes on first anymore.

Roger

http://home.insightbb.com/~derbyrm

"Brian D" wrote in message
. ..
Makes sense, but note that the gap can also be filled with thickened
epoxy mixes. If you are being religious about it, you'd use glass
microfibers plus silica (to make the mix nonsagging.) If you are going
with the wet-on-wet approach, then the outer layers of glass tend to
press down those selvage edges and what's not entirely pressed down will
typically fill with epoxy anyway ...and you can spot and fill bubbles
(syringe) after curing as well. But if allowing each layer of tape to
cure prior to applying the next, then the thing to do is to use a carbide
scraper (plus light sanding) to taper the selvage edge nicely AND fill
the remaining 'gap' (as you call it) with thickened epoxy. Most people
will just use phenolic microballoon or glass microsphere mixes for this,
but like I said, if you were entirely anal about it, you'd use glass
minifibers and silica to fill the gap with something that has a bit more
tensile strength. But IMHO, using glass minifibers and silica would be
ENTIRELY unnecessary. If the structure is weak enough to gain from such
techniques, then you've already blown it and should re-think your
engineering anyway.

Brian D


"derbyrm" wrote in message
m...
For strength, there needs to be a high ratio of glass to epoxy, thus one
squeegies (sp?) out as much resin as possible. If the selved edge of
the narrow piece creates a step that wasn't sanded down, the wider layer
of glass will bridge the step and the space under the bridge will be
filled with air. That's why the plastic sheet overlay is so effective.

Roger

http://home.insightbb.com/~derbyrm

wrote in message
ups.com...
cavelamb wrote:
(corrected to bottom posting to preserve teh thread)

If you go from the smallest piece first you have a chance of
trapping
concentric rings or squares of bubbles in the laminate. Sam


wrote:

Can you explain a bit more on your observation of this problem? I
don't quite follow you.

Thanks.

Jay Chan


You need to try it to see for yourself.

Bubbles are bad.

The are air spaces inside the laminate.

Not good for dimensional stability - or strength

But why putting the smallest piece first will increase the chance of
having air bubbles inside the laminate?

Jay Chan

dog wrote:
This is rather bad advice. In almost all cases, you WANT THE RESIN AND
PATCH TO BOND TO THE CORE. Putting plastic between the core and the
resin leaves a void too, just not one filled with gasses. Most cores
DO NOT react to Epoxy by dissolving and gassing off... especially if
the core is a good end-grain balsa, which it is on the better boats.

Not tostart an argument, but there are many foams to choose from and
some are lighter & stronger than balsa.

... If the core is not bonded to the repaired area, you have
effectively created a section that is pre-delaminated.

Yeah, that was kind of what I thought too. But you have to keep in
mind, everybody wants something different.

DSK

Yes, there are many foams to choose from, but few have the same
characteristics that make end-grain balsa such a good core material.
Few of the foams bond and wick epoxy resin as well as end-grain balsa.
Few of the foams have the compressive and sheer strength and resistance
that end-grain balsa has. Few of the foams have the high temperature
resistance to softening and deforming that balsa has. To date, nothing
that I have seen is better than a good end-grain balsa for most
applications. There are a few high-tech honeycomb materials that are
almost as good, but they're generally far more expensive and difficult
to work with than end-grain balsa.

dog wrote:
Yes, there are many foams to choose from, but few have the same
characteristics that make end-grain balsa such a good core material.
Few of the foams bond and wick epoxy resin as well as end-grain balsa.

Are you looking for bond strength / peel resistance? Some
foams are better in this regard.


Few of the foams have the compressive and sheer strength and resistance
that end-grain balsa has.

Now that's just plain wrong, it is very easy to find foam
that is better in these characteristics. The question is,
can you find better foam that is still lighter, and also
doesn't cost a lot more?

Look into the specs on foam in the 20# and up range.


... Few of the foams have the high temperature
resistance to softening and deforming that balsa has.

Considering the temp characteristics of epoxy, I don't think
this makes much difference. OTOH it is possible to use
fire-retardant foams... balsa is wood after all, and will burn.


... To date, nothing
that I have seen is better than a good end-grain balsa for most
applications.

That statement leads me to wonder how far you've looked.

... There are a few high-tech honeycomb materials that are
almost as good, but they're generally far more expensive and difficult
to work with than end-grain balsa.

It's true that balsa is relatively inexpensive and easy to
work with. I like it... but for my ownpersonal super boat
project, I picked a type of foam that a friend used in
airplane kits. Has some drawbacks, but I have no doubt that
it will be lighter and stronger than balsa... and it won't
ever ever ever rot.

Fresh Breezes- Doug King

Brian D wrote:
You mean like using a boat as a male form and the shrink wrap plastic as a
mold release? Boats have been used as male forms before, but I can't say
that I've seen anybody using plastic as a mold release.

Done it myself. Works like a champ as long as the hull
doesn't have any re-curve that locks the new la-up to the
old hull.

.... Generally, the
lay-up needs to be flexible or in more than one part however, else you may
not get it off the form. Mold release or not.

Bingo!

... Be sure to take pix!


And post links to them here. It always makes the day a
little better to see what other people are up to.

DSK

dog wrote:

Yes, there are many foams to choose from, but few have the same
characteristics that make end-grain balsa such a good core material.
Few of the foams bond and wick epoxy resin as well as end-grain balsa.

What a crock of crap.

The only time someone uses balsa as a core material on a boat is
because they don't know any better or can't afford anything else.

Lew

What Lew said.

"Lew Hodgett" wrote in message
ink.net...
dog wrote:

Yes, there are many foams to choose from, but few have the same
characteristics that make end-grain balsa such a good core material.
Few of the foams bond and wick epoxy resin as well as end-grain balsa.

What a crock of crap.

The only time someone uses balsa as a core material on a boat is
because they don't know any better or can't afford anything else.

Lew

Jim Conlin wrote:
What Lew said.


What I like is the tactful way he says it

DSK