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West Wight Potter Mailing List maintainer
dfarrell@ridgecrest.ca.us
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Eric,
I'm sending this to the whole list, since nobody else replied to your
earlier request for advice.
West Systems/Gougeon Brothers has a book "Fiberlass Boat REpair and
Maintenance" which covers this type of repair in detail. The book is
available at West Marina and other places selling West System Epoxy for
$3.50. I highly recommend it. Chapter three is specifically about repairing
stringers and floors.
You've got the right idea, but you want to lay down the layers of cloth with
the widest one at the bottom, with successively narrower ones as you add
layers. Each layer should be about 1" narrower than the last. The purpose
of this is to avoid "hard spots" where the laminate ends; hard spots
concentrate all the force along one line, and can cause flexing of the hull
at the edge of the new glass (causing gelcoat cracks, and possibly, hull
cracks too). The final, top (smallest) layer should extend at least 2" on
either side of the tube (forward and aft) onto the hull, to provide enough
area for adequate adhesion.
You also need to do the same "tapering" of the laminate thickness at
outboard edges of the repaired area, where it contacts the old stringer, so
you don't cause cracks in the stringer at the edge of the repaired area.
FABRIC PREP: Measure the thickness of the skin on the stringer. It may be
thiner on sides than top. The top is what counts, since that's where the
greatest strain/compression is.
You'll want to duplicate the same thickness, or go a teensy bit thicker.
Using 22 oz cloth, you get about .40" per layer, so you can figure out how
many layers you need. There's no difference between 4 layers of 22 oz and 8
layers of 11 oz cloth in terms of strength. (12 oz cloth gives about .20"
thickness per layer)
Remember that the final (top and smallest) layer should extend 2" over the
sides of the tube, over the hull. It'll take some figuring to get the right
numbers of layers and have the last layer 2" wider than the tube.
Lay the glass down, Fibers parallel to the stringer, and draw where to cut
it. Cut it and check your fit and cut it. Trim and shape it to fit any
curves without buckling. Cut successive layers 1" smaller on all the
edges, so you get a tapered thickness when they're all installed. The
smallest piece of cloth should contact about 2" of the hull at the front and
back.
Trace around the outside of the largest piece with a waterproof marker, so
you know where to lay the first piece.
If you want to go high tech, you can sandwich in a layer of high-tech
unidirectional glass on the tops and side of the stringer, halfway through
the laminating process with the regular cloth. Lay the fibers oriented
parallel to the load (from side to side). But that's probably overkill :)
SURFACE PREP: Sand and wash (with soapy water) the area to be repaired to
remove any epoxy amine that might have been used in the original string
layup (if epoxy was used originally). Wipe with acetone and lots of clean
rags to remove any wax (if polyester was originally used). 50 grit on a
sanding wheel or 100 grit by hand is good for roughing to provide surface
imperfections for the new epxoy to "key" into mechanically.
Grind the cloth over the crack in the stringer to about a 12:1 bevel towards
the outboard sides. for example, if the cloth is 1/8" thick, bevel it to at
least 1/8" x 12 = 1-1/4" outboard on each side. Right over the break, grind
it down as thin as you can without getting into the tube.
(I'm assuming you have a hollow tube for a stringer. This is called an
"inactive core stringer". The strength comes from the cloth, and obviously
not from the tube, so if you nick the tube, don't worry. Just leave it
intact enough to use as a form to give you a smooth shape. You don't want
any gross irregularities in the shape or else the repair may flex and break
right there.)
Since it cracked once there, I probably would make a wider bevel and put
down a slightly thicker laminated layer of cloth. But keep in mind that you
must taper the width of the repair whereever it contacts the stringer, or
you may cause a new crack at the edge of the repair. Increase the thickness
just one or two layers of glass -- doubling the thickness of the laminate
increases strength by 8 times, so there's no need to use a lot more layers.
APPLYING THE FIBERGLASS SKIN:
If the joint of the tube and hull forms a hard right angle, apply a line of
thickened expoxy to the joint and then smooth with the end of a rounded
tounge depressor to form a 1/2" raduis'd fillet. the glass wont adhere well
to a sharp corner, so you fill it in and make a gradual curve for the glass
to form against. Cure until rubberly(don't let it fully cure, or you'll
have to sand it before applying the first layer of cloth)
Squeegee a thin layer of of epoxy and colloidal or high density thickener on
the surface (consistencey of mayonaise), filling in any surface
irregularities. Apply the first layer (biggest) of cloth, and
squeege/roller it down. Use a clean wooly roller to wet the surface
completely with resin for the next layer; don't move/snag/drag the first
layer. Lay the next piece of cloth, wet it with resin ,squeegee/roller out
excess; lay the next piece of cloth, etc. Make sure you saturate each layer
completely, but squeegee/roller out any excess resin or else the subsequent
layer will slide around as you add more layers.
Allow the whole repair to reach intial cure (rubbery, soft enough to dent
with a fingernail). Apply two or three more layers of epoxy. You can tint
these last layers of epxoy resin with white pigment, if you wish to blend it
in with the surrounding finish.
============= That's it in a nutshell. You probably won't be able to work
out the dimensions of the glass layers and tapering and adhesion areas
exactly according to the rules, but you'll be able to come close enough for
a professional quality repair.
The book is much easier to understand than my explanation, I'm sure, and it
has pictures. If I had my scanner working, I could send you Chapter three
from the book, but the old computer is dead and I haven't gotten the new one
up and running.
I'm assuming you've worked with epoxy before, so you know how basic
techniques -- what initail cure vs final cure means, how to pick the right
catalyst for the ambient temp, use a repirator and fan, squeegee and metal
roller, use a foam roller specifically for epxoy (not a house painting
roller) nitrile gloves (the blue or green ones), etc.
If you're not already familiar with all this, you really MUST get the book
and read the introductory chapter on fiberglass construction and Chapter 9
on how to use epoxy.
Best of luck,
Judy
> -----Original Message-----
> From: etj@nwlink.com [mailto:etj@nwlink.com]
> Sent: Thursday, February 10, 2000 6:53 PM
> To: Judith Franklin Blumhorst
> Subject: P19 problem
>
>
> Judy, did you ever see my email about a problem I had with my
> P19? One of the
> semi-cylindrical stiffening braces under the v-berth seat is
> split right where
> the port and starboard parts of it meet. any suggestions on how
> to fix? I figure
> I can just sand down the surrounding area and layup glass over
> it, in succeedingly
> larger pieces for good adhesion. Any thoughts?
> http://www.nwlink.com
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