Blister Repair
by Don Casey
Fiberglass
blisters occur because water passes through the gelcoat. Water soluble
chemicals inside the laminate exert an osmotic pull on water outside,
and some water molecules find a way through the gelcoat. As more water
is attracted into the enclosed space, internal pressure builds. The
water molecules aren't squirted back out the way they came in because
they have combined with the attracting chemicals into a solution with
a larger molecular structure. Instead, the pressure pushes the covering
gelcoat into a dome-a blister.
There has been a great deal of hysteria about blisters, but the reality
is that the number of boats that develop serious blister problems is
extremely small. An occasional blister or two is not a serious problem,
any more than is an occasional gouge in the hull. Some boats seem to
exhibit a greater propensity to blister, presumably due to the chemical
components used and/or the layup schedule, but all boats are at some
risk. Surveys suggest that about one boat in four develops blisters.
Repair
materials
Effecting the repair of a few hull blisters requires an appropriate
quantity of epoxy resin and hardener. Do not use polyester resin for
blister repairs; you need the stronger adhesion and better water impermeability
epoxy provides.
You also need a filler to thicken the epoxy into a putty. Select colloidal
silica. Never use microballoons or any other hollow or absorbent (talc,
for example) fairing compound to fill blisters.
A quart of acetone, a box of TSP (trisodium phosphate), a few acid brushes,
and a 36-grit sanding disk completes your supply list. If the blisters
penetrate the laminate, you may also need a yard of 6 to 10 ounce fiberglass
cloth.
Minor
Blistering
The first step in minor-blister repair is opening the blister to let
it drain. Pop the dome with a chisel, screwdriver, or rotary tool. Be
sure you are wearing eye protection; pressure inside a hull blister
can be double that of a bottle of champagne, and the fluid that blasts
out when you pop it is acid
Load a disk grinder with your 36-grit disk and grind the open blister
into a shallow depression. The rule of thumb is that the depression
should be 20 times as wide as it is deep, and it should only be as deep
as required to remove any damaged laminate beneath the gelcoat.
Use a plastic mallet or the handle of a screwdriver to tap the hull
all around the blister. Sound laminate will give a sharp report. A dull
or flat sound anywhere indicates additional delamination, meaning that
the blister is larger than you thought. Increase the circumference (not
the depth) of the depression until the laminate all around it is sound.
Flush the open blister with water, then scrub it squeaky clean with
a solution of hot water (if available) and TSP-about a quarter cup of
TSP to a gallon of water. Rinse thoroughly, then allow the blister to
dry for at least 48 hours, longer if practical. If you dry-store your
boat for the winter, grind and scrub blisters at haulout but don't fill
them until launch time.
Filling
Just before filling, scrub each depression briskly with a clean rag
dampened with acetone.
Mix a small quantity of epoxy (one pump) and paint this unthickened
resin into the cavity. Wet out the entire surface of the depression.
Use an acid brush to apply the epoxy and give this application 20 or
30 minutes to begin to kick. For shallow blisters, prepare a small amount
of fresh epoxy (one or two pumps) and thicken it to peanut butter consistency
with colloidal silica. Fill the depression completely with this mix,
using a squeegee to compress and fair the filler. Silica-thickened epoxy
is difficult to sand, so take extra time to fair the epoxy as well as
possible while it is wet.
Deep blisters require the replacement of the damaged glass fabric. Cut
a disk of fiberglass cloth the size of the bottom of the depression,
then cut several more, each a little larger than the last. Use only
cloth; never use fiberglass mat with epoxy resin.
Wet the bottom of the cavity with epoxy and lay in the smallest disk
of cloth. Wet out the cloth with resin until it is transparent, then
lay in the next, slightly larger disk. Wet this one out, using the end
of the brush to tamp the disks and compress them together. Continue
adding disks and saturating them with resin until the repair is even
with the surrounding surface.
Whether you have filled the cavity with glass disks or epoxy putty,
allow the filler to kick. When the epoxy is no longer fluid, but still
tacky, paint the repair and an inch or so beyond with at least two coats
of unthickened epoxy, letting each coat kick before applying the next.
Let the repair cure for 24 hours, then scrub it with water and an abrasive
pad (like Scotchbrite) to remove the waxy film on the surface of the
epoxy. Fair the repair with a sanding block and you are finished.
Gelcoat should never be applied over epoxy. Since the repair will be
covered with bottom paint, there is no need for a gelcoat surface anyway.
Don't use gelcoat in blister repair.
Boat
Pox
Boat pox is a much more serious condition, related to the occasional
blister like acne to the occasional pimple. If the bottom of your boat
is covered with blisters, filling them won't cure the problem. Pox is
a systemic condition indicating that the hull is saturated. The actions
necessary to remedy boat pox require specialized equipment and expertise.
For more
information about hull-damage repair, consult Sailboat Hull & Deck
Repair by Don Casey.
|
