Confusion: Epoxy or polyester?
 

Hi,

Once again I need to draw on the wisdom of this group hoping for an
explanation or clarification. I posted earlier regarding the repair of an
impact hole in a fiberglass sailboat hull. I'm using the Don Casey boook
"Sailboat Hull & Deck Repair" and have run into conflicting/confusing
information.

In the book, he states that epoxy resin should always be used for repair
because it creates a stronger bond than polyester resin. He then goes on to
state that the only time not to use epoxy is when using polyester gelcoat.
Okay, so far so good.

Then in the section titled "Impact Damage" which is what I'm following, the
first step in the actual repair is to lay up a piece of acrylic on the
outside of the hull and apply a 20 mil coat of gelcoat from the interior up
against the acrylic, then start laying up the fiberglass layers on top of
the gelcoat.

This is a large repair (4' long by approx 4 " h) and strength is the primary
importance to me, so it would seem epoxy is what I would want to use. But
following those directions I have to use polyester resin to have the
fiberglass adhere to the first gelcoat layer. Is there something I can use
to substitute for the gelcoat that will allow me to use epoxy? Any
suggestions? Is there some kind of paint that will have a thick enough
consistency to use on a nearly vertical surface? Using epoxy resin for the
strength is more important to me than what the finish looks like, it'll be
underwater so the cosmetic aspect doesn't matter much to me.

Thank you for any help and suggestions on how to handle this part. Also, one
person requested I keep posting the progress of this disaste....I meqan
project. I'm putting together a web site of sorts to show the progress.
www.geocities.com/lokisdad1026 if you're interested. I have little time and
no knowledge of web page design so don't expect much but maybe it'll help
someone so I'll do the best I can there.

Mike

Mike,

Fortunately, you got about 99% good information. Gelcoat is polyester
resin with color in it (actuate, but somewhat simplistic description).
It does not bond well to unprepared epoxy, but it (like any polyester)
doesn't bond particularly well to unprepared polyester either. This
least you with two choices - 1. sand the epoxy before appying the
gelcoat (unless you are real good, you probably will anyway) or - 2.
apply a coat of vinyl ester resin to the epoxy to be a primer for the
gelcoat. I often end up doing both.

Get and use epoxy for the repair. Get the pump set so you don't have to
worry about mix-measurement. Get the good stuff and everytime you have
think about what it is coating - think about what a boat yard would be
charging you.

Good luck

Matt Colie

Bonasa wrote:
Hi,

Once again I need to draw on the wisdom of this group hoping for an
explanation or clarification. I posted earlier regarding the repair of an
impact hole in a fiberglass sailboat hull. I'm using the Don Casey boook
"Sailboat Hull & Deck Repair" and have run into conflicting/confusing
information.

In the book, he states that epoxy resin should always be used for repair
because it creates a stronger bond than polyester resin. He then goes on to
state that the only time not to use epoxy is when using polyester gelcoat.
Okay, so far so good.

Then in the section titled "Impact Damage" which is what I'm following, the
first step in the actual repair is to lay up a piece of acrylic on the
outside of the hull and apply a 20 mil coat of gelcoat from the interior up
against the acrylic, then start laying up the fiberglass layers on top of
the gelcoat.

This is a large repair (4' long by approx 4 " h) and strength is the primary
importance to me, so it would seem epoxy is what I would want to use. But
following those directions I have to use polyester resin to have the
fiberglass adhere to the first gelcoat layer. Is there something I can use
to substitute for the gelcoat that will allow me to use epoxy? Any
suggestions? Is there some kind of paint that will have a thick enough
consistency to use on a nearly vertical surface? Using epoxy resin for the
strength is more important to me than what the finish looks like, it'll be
underwater so the cosmetic aspect doesn't matter much to me.

Thank you for any help and suggestions on how to handle this part. Also, one
person requested I keep posting the progress of this disaste....I meqan
project. I'm putting together a web site of sorts to show the progress.
www.geocities.com/lokisdad1026 if you're interested. I have little time and
no knowledge of web page design so don't expect much but maybe it'll help
someone so I'll do the best I can there.

Mike

On Sun, 12 Sep 2004 10:53:40 -0600, "Bonasa" wrote:
Then in the section titled "Impact Damage" which is what I'm following, the
first step in the actual repair is to lay up a piece of acrylic on the
outside of the hull and apply a 20 mil coat of gelcoat from the interior up
against the acrylic, then start laying up the fiberglass layers on top of
the gelcoat.

=========================================

For a first time repair attempt, I would not personally recommend that
approach although it could be made to work. I'd use epoxy because of
it's stronger secondary bond, do the work from either inside the hull,
outside, or both depending on what makes the most sense structurally.

If the hull is substantially weakened and flexible in the area of the
repair, then you need to temporarily attach a backing material either
inside or outside to provide stability during the early stages of the
new lay up. This could be something as simple as 1/4 inch plywood
covered with PVC garbage bags and attached with temporary screws
through the hull. The temporary backing can be removed after the first
few layers of glass have cured and the screw holes filled with
thickened epoxy.

On Sun, 12 Sep 2004 10:53:40 -0600, Bonasa wrote:

Hi,

Once again I need to draw on the wisdom of this group hoping for an
explanation or clarification. I posted earlier regarding the repair of an
impact hole in a fiberglass sailboat hull. I'm using the Don Casey boook
"Sailboat Hull & Deck Repair" and have run into conflicting/confusing
information.

In the book, he states that epoxy resin should always be used for repair
because it creates a stronger bond than polyester resin. He then goes on to
state that the only time not to use epoxy is when using polyester gelcoat.
Okay, so far so good.

Then in the section titled "Impact Damage" which is what I'm following, the
first step in the actual repair is to lay up a piece of acrylic on the
outside of the hull and apply a 20 mil coat of gelcoat from the interior up
against the acrylic, then start laying up the fiberglass layers on top of
the gelcoat.

This is a large repair (4' long by approx 4 " h) and strength is the primary
importance to me, so it would seem epoxy is what I would want to use. But
following those directions I have to use polyester resin to have the
fiberglass adhere to the first gelcoat layer. Is there something I can use
to substitute for the gelcoat that will allow me to use epoxy? Any
suggestions? Is there some kind of paint that will have a thick enough
consistency to use on a nearly vertical surface? Using epoxy resin for the
strength is more important to me than what the finish looks like, it'll be
underwater so the cosmetic aspect doesn't matter much to me.

Thank you for any help and suggestions on how to handle this part. Also, one
person requested I keep posting the progress of this disaste....I meqan
project. I'm putting together a web site of sorts to show the progress.
www.geocities.com/lokisdad1026 if you're interested. I have little time and
no knowledge of web page design so don't expect much but maybe it'll help
someone so I'll do the best I can there.

Mike

I think gelcoat is just polyester resin. Maybe you don't really need the
gelcoat?

Maybe you can use an epoxy fairing compound (or epoxy thickened with an
appropriate filler) instead of the gelcoat. Or just make the repair, and
fair the boat afterwards (with thickened epoxy).

Just make sure your repair doesn't protrude out past the fair line. That
will make the boat impossible to fair.

Also, keep in mind that it is a lot of work to sand or grind off unwanted
fiberglass/resin, but fairing compounds are designed to sand easily.

--Mac

Before epoxy came along with it's excessive marketing (they all buy the
same stuff, add a few things to make it "special", and spend a lot of
money trying to convince buyers that their product is marginally better),
polyester was being sucessfuly used to repair poylester boats for decades.
The first book I read on fibreglass boat repair said a polyester repair is
stronger than the original. Epoxy may be stronger than polyester (both the
bond and the resin itself) but you don't need it. For more strength you
can use additional layers of polyester. Use epoxy to build a boat which is
as strong as a polyester boat but weighs less overall.

Gelcoat is there to provide protection from the sun. Both polyester and
epoxy will break down if exposed to sunlight for long periods of time and
then neither is very strong. You have to cover exposed epoxy with
something to protect it from the sun. Gelcoat also provides extra
resistence to water penetration because over time polyester can soak up
water. Epoxy is waterproof and will not soak up water unless it's stressed
and develops lots of tiny cracks. Putting a more waterproof epoxy
patch on a hull made with polyester will not do anything to keep the hull from
absorbing moisture it it's going to. Keep the boat's gelcoat in good shape
instead.


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I would have to see the damage, but here is a way to easily do the repair, if
there is any such thing.

Use only polyester, it is stupid to use a stronger material as the rest of the
hull is polyester.

Remove all wet and loose chunks from the hole in the hull, and bevel just the
glass part maybe 20/1 (twenty times the thickness of the layup, core should be
all removed and it can be replaced by a butt joint!) all around the damage on
the inside of the hull , to get a good overlapping bond.

Apply wax and pva (polyvinyl alcohol) release agent to the side of the hull
opposite to the damaged area, and gel coat and make a light polyester layup,
maybe 3/4 oz matt with 6 oz cloth backing.

Remove the layup while is is slightly rubbery, and "pop" it so it has the
opposite curvature of the damaged side.

Cut the layup down so it overlaps the damaged area maybe 1/2 inch Apply wax and
PVA to the layup mold and tape it over the damaged area. It should fit
perfectly if the hull is symetrical. Go inside and paint the damaged area
with gelcoat, after washing down the 20/1 scarf with acetone. Allow the gel
coat to cure, and lay up the outer skin with polyester with a similar build.
Then lay a core material in using polyester made into a paste with
microballoons, and complete the inner skin, if this is a cored hull.

Pop the layup mold off the outside, sand a little and go boating.

Barry Palmer, for Sevtec
Barry Palmer, for A
HREF="http://members.aol.com/sevtec/sev/skmr.html"Sevtec/A

On Mon, 13 Sep 2004 22:52:34 +0000, Barry Palmer wrote:

I would have to see the damage, but here is a way to easily do the repair, if
there is any such thing.

Use only polyester, it is stupid to use a stronger material as the rest of the
hull is polyester.

As I understand it, the issue is not whether polyester resin is strong
enough, but whether new polyester will bond well to already cured
polyester.

I don't have any experience repairing polyester with polyester, but I have
repaired polyester with epoxy and had no problems.

[snip]

--Mac

"Mac writes:
As I understand it, the issue is not whether polyester resin is strong
enough, but whether new polyester will bond well to already cured
polyester.

Bingo!

Polyester is a lousy glue which is why it doesn't make a good repair
material.

It works well for laying up something new, but not later as a repair
material.


--
Lew

S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: http://home.earthlink.net/~lewhodgett for Pictures

"Lew Hodgett" ) writes:

It works well for laying up something new, but not later as a repair
material.

that wouild come under "urban boatbuilding myth" no doubt started by
purveyors of epoxy resin.

I've made polyester repairs to polyester racing hulls - no problem. These
were club-owned boats which took quite a beating. The club had an
unofficial rule "no contact, no protest", nobody wanting to spend their
Sunday afternoons on a protest committee hearing protests, which is why so
many boats required so many repairs. We also didn't like spending weekday
evenings doing fibregalss repairs. There must have been a tradeoff there
somehwere. :)

I can show you a video on boat maintenance and repair available from the
Ottawa public library which shows step-by-step how to repair structural
damage to fibreglass powerboat hulls using polyester resin and fibreglass.



--
------------------------------------------------------------------------------
William R Watt National Capital FreeNet Ottawa's free community network
homepage: www.ncf.ca/~ag384/top.htm
warning: non-FreeNet email must have "notspam" in subject or it's returned

Bonasa wrote:
Hi,

Once again I need to draw on the wisdom of this group hoping for an
explanation or clarification. I posted earlier regarding the repair of an
impact hole in a fiberglass sailboat hull. I'm using the Don Casey boook
"Sailboat Hull & Deck Repair" and have run into conflicting/confusing
information.

In the book, he states that epoxy resin should always be used for repair
because it creates a stronger bond than polyester resin. He then goes on to
state that the only time not to use epoxy is when using polyester gelcoat.
Okay, so far so good.

Then in the section titled "Impact Damage" which is what I'm following, the
first step in the actual repair is to lay up a piece of acrylic on the
outside of the hull and apply a 20 mil coat of gelcoat from the interior up
against the acrylic, then start laying up the fiberglass layers on top of
the gelcoat.

This is a large repair (4' long by approx 4 " h) and strength is the primary
importance to me, so it would seem epoxy is what I would want to use. But
following those directions I have to use polyester resin to have the
fiberglass adhere to the first gelcoat layer. Is there something I can use
to substitute for the gelcoat that will allow me to use epoxy? Any
suggestions? Is there some kind of paint that will have a thick enough
consistency to use on a nearly vertical surface? Using epoxy resin for the
strength is more important to me than what the finish looks like, it'll be
underwater so the cosmetic aspect doesn't matter much to me.

Thank you for any help and suggestions on how to handle this part. Also, one
person requested I keep posting the progress of this disaste....I meqan
project. I'm putting together a web site of sorts to show the progress.
www.geocities.com/lokisdad1026 if you're interested. I have little time and
no knowledge of web page design so don't expect much but maybe it'll help
someone so I'll do the best I can there.

Mike


Aw, lay a coat or two of poly glass on the outside to accept the
matching gel coat. Then complete the repair from the inside with
epoxy, if having a hard spot at that point doesn't bother you.

Epoxy is so strong that it makes the surrounding polyglass look weak
by comparison, but it isn't. Polyglass will stick to properly
cleaned and toothed polyglass well enough that the actual benefit
from using epoxy is not really practically useful, or cost
effective. Overbuild the inner part if it makes you feel safer.

Feather the edges out on the inside and equally on the out. 10:1 is
plenty.

Terry K