Fibreglass repairs to hull small boat

G'day again Nick,

You can use standard resins, the end result will be little heavier than an epoxy job.

Make sure the old glass has a really good key (Rough surface) as the only bond will be via this key.

An epoxy has a stronger grip because it has both a mechanical and chemical bond when cured.

Avagoodweekend......
 
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An epoxy has a stronger grip because it has both a mechanical and chemical bond when cured.

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No, I'm sorry, that's wrong. Epoxy can't make a chemical bond to polyester. it's a totally different chemical. Have a look at the West System website here
This is a short but relevant extract:
"Because of the difference in curing chemistry, it is not possible to achieve a chemical bond between epoxy and polyester gelcoat."
 
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Agree completely. Absolutely nothing wrong with polyester when applied to a polyester substrate - as long as the preparation is sound (the preparation also needs to be sound if you choose epoxy).

Cut back the surface and then clean liberally with acetone. The acetone not only cleans the resin surface but softens it enough to help make a chemical bond. After cleaning with acetone (thoroughly) touch it with your fingers - it should feel tacky.

Epoxy is a better adhesive than polyester, no question, but it relies solely on making a mechanical bond.

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As has been said. No need to use epoxy for this sort of repair. Polyester poperly repaired will do a more than adequate job and be much less expensive.

You are only repairing polyester not trying to bond to other materials.

Iain
 
polyester it is then. the repair doesn't have to look perfect. most of this year will be spent on the chelmer with the occasional trip through the sea locks in a very calm day. i'll put a few layers on the bottom and top and finish of with a new coat of paint all round. you have all been most helpful.
kind regards nick
 
G'day Mono,

<<Because of the difference in curing chemistry, it is not possible to achieve a chemical bond between epoxy and polyester gelcoat.>>

Perhaps this should have read:
<<Because of the difference in curing chemistry, it is not possible to achieve a chemical bond between epoxy and polyester gelcoat when applying the polyester over an epoxy.>>

The extract you quoted (above) from the site actually refers to sticking Gelcoat to epoxy, not the other way round.

Also note the heading at the top of the page "Polyester over epoxy"

There is no secondary bond when adding standard resins to standard resins.
There is a secondary bond when applying epoxy over standard resins.

Avagoodweekend......
 
If you can show evidence that epoxy will bond chemically with polyester I'd be very interested in seeing it. My understanding is that epoxy only bonds chemically with uncured epoxy - everything else is a mechanical bond.

I'm not knocking epoxy; it's a fantastic adhesive and I use it frequently. But it's horses for courses and for this sort of repair I would always use polyester.
 
iv'e made a shopping list at www.cfsnet.co.uk
Resin Brush 50mm (2")
2kg Roll CSM 300gm is 6sq mtrs
3kg pack General Purpose Resin and catalyst
3 Square Metre Roll Surface Tissue 30gm
Calibrated Mixing Cup
Bucket 1 litre
Acetone 1 litre
Surgical Latex Gloves

this was a list they compiled when i told them what i needed to do. do i wait for the first layer to cure before adding another layer of put on second laer when resin is still tacky and does this go for the final tissue layer or do i flatten out the csm layers and wait to dry and sand flattish before i apply the tissue layer.
 
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iv'e made a shopping list at www.cfsnet.co.uk
Resin Brush 50mm (2")
2kg Roll CSM 300gm is 6sq mtrs
3kg pack General Purpose Resin and catalyst
3 Square Metre Roll Surface Tissue 30gm
Calibrated Mixing Cup
Bucket 1 litre
Acetone 1 litre
Surgical Latex Gloves


[/ QUOTE ]

Holy moses! Are you repairing a nine inch crack or a nine foot one!

6 sq m of cloth! thats about 70 square feet!

You should need no more than 1 square meter of cloth and about 500gm of resin.

you will need 5 or 6 1" brushes and 2 or 3 mixing jars/pots ( coffe tins, jam jars ). You are going to do the job in a couple of sessions.

The shelf life of polyester resin is around 20 minutes when mixed in a warm garage. Mix up a kilo at a time and in 1/2 an hour you will have a smoking hot tub of clear concrete!
 
it does seem i've gone a bit overboard doesn't it.
i was just thinking whilst its out the water , to stop it happening again i would put 2 layers on each side along the crack but go the whole lenght of the keel area and about 10 ins either side of it so it's really strong.
i think i'll leave the finishing cloth because i can paint over the fibreglass inside and out.
 
this is what i plan to order today

1kg polyester Resin
1kg Roll CSM 300gm is over 3sq mtrs
Resin Brush (1.5")x4
Syringe 10ml
Paddle Roller 70mm x 21mm for expelling air trapped
it comes to £28.00 inc delivery.
i think the repair will be strong enough with this lot. i'll decide on the finish later but i think i'll sand and paint it.
 
It looks as though you've dropped the idea of a tissue layer. I'd agree with that - it won't add to the strength of the repair. 300gms are ok though my personal preference is for 450gms. As you get used to handling the materials you start to look for ways of speeding the process up and 450 gms means less work!

The roller is important - not just for expelling air but also to drive out excess resin. You should aim to use as little resin as possible - just make sure that the fibres are properly wetted out. A resin rich laminate is weak.
 
i repaired my little dinghy last summer and the repair fell out after its first soaking as the csm was soaked with resin and way too much hardener. i put several layers of resin rich csm on and they all fell out asap
 
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