From old NG post of howto.

Yes boring I agree but hey it's on topic & who knows might even get a
boating discussion going:-)

Bill,

I guess you are aware this is a bit of a big job, subject to numerous
ifs, maybes & buts,so on the assumption you wish to do it yourself for
whatever reason, this is a NG you asked so I'll give it a go.

I'm sure you'll get lots of other advice. I'll be making certain
assumptions in this like, that you haven't done this before & have no
fibreglass experience, the boat is made of polyester, not vinyl ester or
epoxy, that the laminate is no more than 1/2 an inch thick, that you
cannot get to the "inside" without considerable destruction/effort &
that you can keep your work area, if not the whole boat, dry & clean,
that said here goes....;

1) You'll need;
a grinder (say 4 inch),
flat foot sander (orbital?),
various cheap disposable paint brushes (wood not plastic handles),
some disposable containers (2 litre ice cream type),
rubber gloves (washing up &/or disposable)
a fibreglass roller (serrated),
Stanley type knife (box cutter),
some waxed, promoted, polyester resin (not much about 4 ltrs should be
plenty it doesn't keep over time so extra will get wasted),
enough catalyst (hardener) for the resin,
enough glass to affect the repair (450gsm chopped strand mat & 900gsm
woven cloth),
a small amount of micro balloons (filler powder, a common brand is Q cells),
a small amount of "gel coat" the same or almost the same colour,
( Don't buy the materials until after step 2 then the resin supplier
will guide you as to the amounts of catalyst, usually 1% by WEIGHT but
don't be too worried & he should be able to help with the gel coat
amount & colour match, make some good recommendations about glass weight
etc, however remember their job is to "sell" you as much as they can:-)),
1 ltr of acetone,
grit paper for the flat shoe sander, as follows 36 grit, 80 grit, 120
grit, then some 400 & 1200 grade wet & dry,

All the tools can be cheapos if you wish & nothing "special" is
required. Don't buy the materials from a "boat" or dealer place they
can't give you proper advice & will over charge you, get it from a
commercial fibreglass chemical supplier)

2) You need to grind the area around the crack into a very gentle
valley, with the bottom of the crack at the bottom of the valley. The
taper should be at "LEAST" 5 times the thickness of the laminate, (i.e.
1/2 inch thick so grind a taper ending 2 1/2 inches from the bottom of
the crack) now your 12 inch crack is a shallow "v" 5 inches wide &
17 inches long. Keep it dry & clean.

3) You now need to get your materials & cut the pieces of glass, stack
them in the order you will lay them. (narrowest/shortest on top). For
the repair you will need to build up from the bottom of the valley in
various layers of glass mixed with resin, each slightly larger (wider &
longer) than the one under it, (i.e each new layer has it's edges
attached to the original laminate & not just put on top of the one
under). The bottom (narrowest/shortest) layer should be 450gsm chopped
strand mat (CSM) then a slightly wider & longer layer of 900gsm woven
cloth (WR) then CSM, WR, CSM, WR & so on till the valley is filled.

N.B. The WR layers MUST always have a layer of CSM between them & the
next (i.e. No WR on
WR you can have CSM on CSM but WR must always be separated by CSM) To
help judge the
number of layers required, each layer of the 450gsm CSM will roll down &
cure to about 1mm
thick, the 900gsm WR about 1.5mm.

4) OK get a SMALL amount of the resin, & thoroughly mix about 1%
catalyst with it in an ice cream container, note the time. Sit & wait,
it should harden in about 20-40 mins if not do it again with a LITTLE
more catalyst or if it hardened in much under 20 mins a
little less. Take a note of the amount, that is what you will use in
your area, temp.brands of resins etc. It doesn't matter much what the
amount is so long as you get gel starting in about 20mins. Once you have
determined "your" mix make sure you stay consistent. (use an egg timer
to remind me when the batch is about to gel)

5) Mix a small amount of resin with hardener & paint the centre of the
valley, then put your first piece (narrowest/shortest) of CSM in, now
roll it gently with your serrated metal fibreglass roller to get all the
bubbles out, don't rush it, it just takes time (at least 5 mins) for the
resin to fully soak through the glass & dissolve its binders, resist the
temptation to speed things up by adding too much more resin, it isn't
required & will weaken the finished repair (strongest is lots of glass
with as little as possible resin). When the first layer is fully "wetted
out" (goes clear) add a little more resin with the brush, not too much,
then put the first layer of WR on & again gently roll all the bubbles
out (it's much easier to wet out than the CSM), then the next layer of
CSM & so on up to about the 5th layer, then stop, wash your brush in
acetone & have a cup of tea, this is important as tea always is:-) if
you put too much on in one go it builds up heat & will weaken the
finished product. When the first 5 layers have cured & don't feel warm
to the touch continue again (important; if for some reason there is a
long delay, say overnight, lightly grind the surface before
recommencing, the wax in the resin comes to the surface & prevents
subsequent layers properly bonding)

6) When the valley is "almost" full & the glass has cured, give it a
good sand with the 36 grit on the flat shoe sander so it is definitely a
little lower (1mm give or take) than the surrounding undamaged hull.

7) Mix some micro balloons (any brand) in resin to make a filler paste,
put in the catalyst & spread it over the area. Time it & you will notice
that the filler becomes about the consistency of cheese about 5mins
before it actually cures hard, this is when you sand it with the flat
foot & 80 grit. (it is soft & very easy to "shape" at this stage).
Repeat, only this time sand with 120 grit, you may need to do this
several times until it is the right shape & surface finish. (important;
If you miss that 5 min window when the resin is cheesy, grind it & apply
paste again, once allowed to cure you'll never get a fair shape into it)

8) Put the gel coat on as thick as you can without sags or runs, when
cured again with the flat foot sander & now the 400 wet & dry, sand it
with a little water, obviously for safety, if your sander is electric be
very careful & just make the surface damp not running wet. The wet & dry
paper will unclog & become reusable after a few minutes soaking in a
bucket of water. When the surface is completely sanded, (i.e. a flat
finish with absolutely no shine anywhere), you can now finish sand it
with a sanding block, lots of water & the 1200 wet & dry, a buff with a
polisher/polish & it's done.

If you could have got to the inside it's much the same except you have
two shallow valleys meeting half way through the laminate, do the inside
first, then the outside.

My apologies for lapsing back & forth from metric to imperial but
that's what happens. gsm= grams per sq. mtr, 3mm= 1/8in. Say 450 gsm CSM
is usually the same as 1.5 oz/sq ft etc.

Karen Smith.