"Soaking" Epoxies - not so good
Generally the thinners based products were a precursor to the full high
solids epoxy catastrophe (and I mean that in the most positive sense).
The most common in Oz is the dear old Epicraft Everdure
which we all used to seal the insides and underdecks and often under
the paint of our racing dinghies in the seventies and very early 80s.
With racing dinghies weight is seen as somewhat critical - an extra
pound or two is considered a catastrophe. Anyway, everdure
was not particularly effective at keeping the water out of the timber -
the boats would still put on weight - this could range from a pound or
two up to about 5% of the hullweight. The reason as far as I have been
able to gather is that the thinners are quite long complex molecules
and as they come out of the epoxy matrix they leave holes behind.
As the thinners content
of Everdure is very high - about 70
percent from memory the holes are very extensive.
Water molecules are
somewhat smaller than the organic molecules so
have little problem going up the same holes and into the wood.
As
the graph below shows - the water uptake of timber treated with
"penetrating" epoxy is about 5 times that of the Modern Epoxy solution
below. A massive 35% compared to less than 8% for the "high
solids" type.
The
Modern Epoxies - a better solution When WEST Epoxy first
came on the scene (there were other good quality
epoxies around at the same time - like the OZ Bote Cote and a few
others ) it first gained popularity in the Tornado catamaran class,
initially in America, where finally it was possible to build boats that
took up very little weight during the sailing season. At the time it
was very much preferred to build in plywood as it provide a much
stiffer boat for the same weight compared to the solid fibreglass
alternative that was allowed under the class rules. Later on foam/glass
sandwich boats were allowed which can be built stiffer still in many
cases.
The good quality
epoxies are termed "high solids". This basically means
that there are almost no thinners added, so there are no wormholes from
the exit of thinners from the matrix - so little opportunity for water
to get in (and not even much exchange of air - no water, no air, no
life - thus no rot if the job is done properly - (all surfaces sealed
with 'pox). This comes with a significant labour and materials cost,
but has a good track record in general.
The objective in coating - Timber
is very unlikely to rot if the moisture content is down around 7 to 10
percent. From the graph above it is clear that the high
solids
epoxy - 105/205 in this case is excellent.
So
we are left with the decision as to how much epoxy to use.
Too
much and you have wasted money - too little and it won't seal the
timber well enough.
In
real life application it is considered that 3 coats are about right.
The first coat will soak in to a large extent so may leave
some
areas a bit dry. The second coat may be absorbed into some
areas
and it is possible you may miss some. The third coat makes
sure
all the dry patches and missed patches are well covered.
Three coats may sound
like a lot of work, but read the page on "Wet
on Wet Epoxy Application" which gives the methods
for saving heaps of labour in applying epoxy in different situations
Identifying High Solids Epoxies
One way to quickly test if your epoxy is high solids is to sniff it.
There should be no intense thinners type smell. Usually the hardener
has a slight ammonia smell, particularly if you have just taken the top
off, but there should be nothing to make you gasp or that has a
thinners type smell.
So West, Bote Cote, System 3 and some others are high solids. Many of
the cheaper "Industrial" epoxies that are available have
thinners added - so won't keep the water out as effectively.
If you contact Epicraft and ask them about the best method to protect
your boat from rot they will sell you their high solids epoxy system -
because they know it is the most effective way of dealing with the
problem. If you start talking about the expense they will be happy to
sell you dear old Everdure and will wish you well
as you depart.
High Solids Epoxies compared. By virtue of being
"high solids" quality boatbuilding epoxies are
excellent moisture barriers. But do they differ in their
physical
properties?
The University of
Queensland did mechanical testing about a decade ago
on a wide range of the epoxies available at the time in Australia and
it showed that
in terms of tensile strength, impact resistance and work to fracture
that you really do get what you pay for. The cheap epoxies are brittle
and not capable of sustaining high loads, whereas the more expensive
ones - Bote Cote, WEST, System 3 are a great deal better. It was
certainly edifying seeing the results from the testing machines
graphed. The good ones were not just 5 or 10 percent better, but often
40 or 50 percent than the cheaper ones.
Can
you make a high solids epoxy soak in better? WEST originally stood
for Wood Epoxy Saturation Technique. But as all
of us who build boats know - if you cut something that has been coated
the epoxy only soaks in a very small distance - a mm or less. So now
WEST stands for Wood Epoxy Stabilisation Technique - which is a much
more accurate description.
However this does not mean that the system is less effective because it
doesn't soak in. WEST and BOTE COTE and other quality boatbuilding
epoxy systems made their reputations because the
stuff works. They provide an effective system for gluing and bonding
timber. That effective system has never relied on soaking very far into
the timber - it simply has to provide an effective bond.
You can thin epoxy to get it to soak in further for gluing purposes,
but the thinners reduce the strength so much that you have made a large
nett loss. They are generally not recommended for gluing applications.
For coating they work, but (here we move very much more into the area
of opinion - ie ... mine) - I have hardly ever found them
necessary.
There are thinners that
you can add to a high solids epoxy that won't
evaporate. The one that Bote Cote do is called TPRDA - which links up
with the epoxy matrix and becomes a permanent part of it. Some of the
other brands have similar stuff.
One area where the TPRDA type thinners (whatever brand) are good is ...
back onto the thread ... where a structural member or surface has a
thin layer of rot. There the thinned epoxy can get in a little distance
and help consolidate the area. However it does not make the timber as
strong as it once was - so if the rot has compromised the structure
then the strength has to be brought up to the original level. So, for
example if you have a timber frame that is 50mm thick and there is a
bit of rot 6mm deep on the bottom corner it MAY be OK to soak it. But
if the frame is 30 or 50% rotten it needs to come out, or be sistered
or a new section scarfed in.
Generally I use Bote Cote for my building, but one of the many good
things that WEST have on offer is a wooden boat restoration booklet. It
is really cheap - about 10 dollars and goes through all the techniques
necessary to restore and repair older boats. One of the most useful
things about it is that it helps you assess the structure to work out
what you need to do.
