This didn't appear to be copyrighted so I lifted it for you. It's pretty comprehensive. It was on a commercial site in Adobe, I would have put in a link but I don't know if allowed.
VOYAGEUR
FYI GUIDE TO ROTOMOLDED
KAYAK & CANOE REPAIR
It’s safe to say that rotomolded polyethylene canoes and
kayaks have revolutionized paddling. Boats made of this
relatively inexpensive material are incredibly durable and
abuse-tolerant as well as near maintenance free. If there
is a downside to “roto” or “poly” boats, it is that should
they be damaged, they are more difficult to repair
compared to their ABS Royalex® and composite
(fiberglass, Kevlar®, etc.) brethren. Indeed, some
polyethylene boats are impossible to repair and some
types of damage pose a major challenge whether it be a
“home” based repair, a shop repair, or even factory
based.
The intent of this brochure is to provide guidelines for
evaluating the repair possibilities and to outline available
repair techniques. It is recommended that you honestly
evaluate the task at hand and determine if it is within your
capabilities or best turned over to a professional.
Damage can result from wear and tear, impact, abrasion,
etc. and can be roughly sorted into cosmetic and/or
structural in nature, although the line between the two
tends to be gray rather than black.
COSMETIC ISSUES & REPAIRS
Cosmetic damage is defined as something that impairs
the boat’s appearance but not its function. On
polyethylene boats cosmetic damage usually takes the
form of abrasion, superficial slits, cuts or gouges, and
localized dents in the hull panels.
ABRASION
Abrasion is basically due to wear and tear and if often
localized. Common locations are on the stems or ends of
the boat and under the seat in kayaks. Much of
polyethylene’s durability comes from its elasticity and
capability to flex and absorb impact and contact. That
flexibility is limited under the seat due to the seat
structure and the concentration of the paddler’s weight.
Abrasion often takes the form of a series of scrapes and
shallow gouges and in most cases doesn’t need to be
attended to unless over time damage continues to
accumulate to the point where gouges become deeper.
This type of damage will transition from cosmetic to
structural when there is a difference in the flexibility
between the abraded section and neighboring hull
sections. Easiest way to test this is to press on the hull
with your palms and compare the resistance.
If you’d simply like to make your hull look better you can
use a sharp knife (x-Acto® type is good) to cut away any
raised edges along sides of abrasion. A file or Surform®
rasp can also be used effectively to smooth out the hull
surface. Unfortunately, painting is not an option as paint
will not adhere to polyethylene.
SLITS & CUTS
Compared to sharp edged river rocks and ledges and
even coarse grained sand, polyethylene is a soft
material. As such, it will come out second best when the
inevitable collision occurs. Abrasion is one result and a
second consequence can be superficial cuts or slits in the
hull. These are primarily noticeable by the raised edges
on either side of the cut. Again isolated cuts are not a
structural concern. To lessen the cosmetic impact, use a
sharp knife, file, or rasp to remove the raised material. A
rotary tool such as a Dremel® or Black & Decker Wizard
® can also be used effectively to remove the feathered
edges.
GOUGES
Gouges are a tough one. At first, it would seem likely that
a gouge could easily be filled and smoothed but
polyethylene doesn’t provided a good bonding surface for
new material and the inherent flexibility of the material
poses a challenge to the bond between original material
and filler or new material. Other than smoothing the
edges of the gouges as you would with slits and cuts, it’s
best to accept and live with it. If you’d like to try to fill the
gouge, you can follow the instructions for filling cracks
included in section on structural repair.
DENTS
Dents can result from impact from paddling or a weight
left resting on the boat. Long term storage in one position
can also produce hull distortions. Prolonged or continual
exposure to sunlight can distort or stress the hull and
create depressed sections of the hull. Tying your boat
down tightly on your roof racks for a lengthy time,
particularly on sunny hot days can result in dents and
deformations. Generally, prevention is the best solution to
this type of damage. Periodically ease your tie down
ropes or straps. Store your boat suspended in web straps
or resting on rigid sections of hull.
Getting an idea of the source of the dents in the hull can
help prevent future damage but doesn’t help deal with
those already in existence. Just as heat is the source for
some dents and deformation, it can also provide the
solution. Polyethylene has some memory and its’ recall
can be encouraged by heat and pressure.
Minor dents can sometimes be removed simply by
leaving the boat exposed to bright sunlight and applying a
gentle pressure on the inside of the hull. If the dent
proves stubborn, you can increase the heat by using a
hair dryer. If still more is needed, a hot air gun may be
suitable but must be used with care. Keep the gun at
least 1” from the hull and in constant motion. Watch
carefully for any signs of glistening or melting of hull
surface. Apply pressure from inside of hull while applying
heat to exterior. Have gloves available to protect your
hands while hull heats.
An alternative approach is to rig a brace to apply
consistent pressure to inside of hull while boat is exposed
to sunlight or heat.
STRUCTURAL REPAIR
Structural damage can affect the hull or such fittings as
hatch rims and the connections between them and the
hull.
A caulk or sealant would seem a likely candidate as a
repair material but unfortunately, polyethylene does not
lend itself to a long-term bond with any sealant. Probably
the caulk that performs best with polyethylene is Lexel ®,
used by many manufacturers to seal the junction
between minicell bulkheads and the interior of the hull.
However, even the best sealant will not adhere well to
polyethylene by itself. Bear in mind that the bond
between bulkhead and hull is reinforced by the
compression of the fitting. The same goes for sealing
****pit rims or similar applications. Caulk or sealant can
make the junction between fitting and hull drier but only if
used as the filling between the fittings, contained and
compressed by mechanical fasteners such as rivets or
buts and bolts.
NOT ALL POLYETHYLENE IS THE SAME…..
First step in evaluating repair possibilities is to determine
whether your boat constructed of crosslink or linear
polyethylene. The terms crosslink and linear refer to the
molecular structure of the material. In years past, most
kayaks were constructed of crosslink polyethylene.
Compared to linear poly, crosslink is stronger and stiffer
but it has its’ disadvantages, one of the biggest of which
is the fact that its’ not repairable. Serious additional
issues are that it is not recyclable and significantly more
hazardous to work with. The development of the new
“super” linear polyethylenes have allowed manufacturers
to abandon the use of crosslink. The performance
capabilities of super linear poly approach those of
crosslink and retain the advantages of their linear origin:
reparability, recyclable, and less hazardous to work with.
It follows then that the first step in assessing the
possibilities of repair is to determine if your boat is linear
or crosslink polyethylene. Unfortunately, the differences
are not readily apparent to the eye. Nor did all
manufacturers shift from crosslink to linear at the same
time. It is far more likely that whitewater kayaks will have
been built of crosslink than touring or recreational boats.
Many manufacturers for a time built their touring and rec
boats from linear while using crosslink for their
whitewater models.
The best means to determine whether your boat is
built of linear or rotomolded polyethylene is to
contact the manufacturer with the boat’s serial
number and ask them to check their production
records
If it is determined that your boat is crosslink, it may be
time to bid a fond farewell to it. You can certainly try to
apply repair techniques for linear polyethylene but
realistically, the boat will never be the same and should
not be subject to the same expectations or the extreme or
hazardous use.
CRACKS
Well, it just goes on and on.
