Fibreglass repairs to hull small boat

N

nyx2k

Well-Known Member
Joined
11 Nov 2005
Messages
805
Location
CHICHESTER
Visit site
i was merrily cruising along the canal yesterday and hit a submerged branch which has caused a small crack right in the bottom of the hull on my 14ft boat. it is now leaking slightly.

i know i have to take it out and fibreglass the bottom but do i do repairs from the inside or outside or both. what type of cloth and resin do i use etc. also what preparation ie sanding or cleaning of the areas to be fixed..

kind regards nick
 
[ QUOTE ]
i was merrily cruising along the canal yesterday and hit a submerged branch which has caused a small crack right in the bottom of the hull on my 14ft boat. it is now leaking slightly.

i know i have to take it out and fibreglass the bottom but do i do repairs from the inside or outside or both. what type of cloth and resin do i use etc. also what preparation ie sanding or cleaning of the areas to be fixed..

kind regards nick

[/ QUOTE ]

Both sides would be best. Remove paint or gelcoat. Any areas of glass that are stressed ( white rather than light brown ) need to be completely removed by cutting out or sanding back to good substrate. Edges of areas to be worked should be chamfered back at an ange of 1:20 or even more. ( this will be hard to acheive on a 14 foot boat since the hull is likely to be only a few mm thick. On a structure that size I would suggest working with 300gm / metre cloth at the most, probably 2 or 3 layers on the outside and whatever is needed to build up the thickness on the inside.

I'd also favour using epoxy over polyester. It sticks like the proverbial whereas polyester to old polyester adhesion is poor.

I'd also stick mostly to hand tools for the trimming back and refinishing once you have laid up the glass. Working with power tools on 2mm thick glass is risky.

Once the patched area is nice and smooth a heavy coat of gelcoat will protect the glass.
 
First question is - are you painting over your repair? I'll assume that as it is an under-water repair you are going to paint over it.

So, I'd get a coarse disc on a sander and give inside and out a good roughing up for a key. As you don't say you actually have a hole but a leaking crack the repair is presumably on a chine which is where a flat bottomed boat is weakest in some ways as the angle of the chine is flexed by shock stresses on the flat panels. If this is the case you can repair and add some beefing up at the same time.

You should firstly take some clean oil cans and a jam jar down to your local glass fibre fabricator, not necessarily a boat yard, look in yellow pages, because it is far cheaper to haggle a price for putting some in your containers than buying small quantities from a retail outlet. Buy resin, hardner in your jam jar, about 1/2 gallon of acetone for cleaning, and some 2 oz chopped strand mat and some glass tissue.

Get a baby feeding syringe from your chemist to add the hardner, plus some disposable plastic or latex surgeon's gloves as well. Get a clean paint brush and some yoghurt pots to mix in. Ask your resin supplier for mix proportions.

Wet out area to be repaired and allow to gell (sticky set not dry). Fresh resin the area, tear off some mat ( tearing allows the repair to be feathered at the edges) and place it on wet resin. Using brush stipple resin into mat until it goes clear indicating it is wetted through. Do not make the mat resin "rich", i.e. too much resin, as the result is weaker. Add a couple more layers and some old rope or paper rope (from your supplier) if you are repairing into an angle of chine and glass two layers over that. Otherwise, three layers on a flattish surface will suffice. Allow to set, a hair drier can speed up the curing process at this time of year by warming the mix.

On the outside, apply two layers as before, finishing up with a couple of coats of tissue to smooth out. When cured, lightly sand to fully feather off, apply another coat of resin and allow to cure. Paint over repair.
 
thats great thanks very much. the leaking area is right next to the keel area. the boat is a dejon 14. it has a flat bottom and the crack is very small but about 9 inches long and it is weeping water but i suppose over a few days it will let in a fair bit of water. i'll take it out over the weekend and start work on it.
 
its been used on the sae all year and i move it to heybridge on the chelmer and it gets a crack in it-- typical.
i suppose i need to let it dry out before i start work on it. the driveway is a hot suntrap so i hope it will dry out fairly quickly.
 
i've decided to do the whole floor of the cuddy area to reinforce the floor and ive worked out the area is 1.5 sq mtrs so if i also do the outside of the floor do i need only one layer or is 2 better being 6sqmtrs. how much resin and hardener will i need for that area.
i presume i need csm mat .
regards nick
 
If you're out of the water already, I doubt very much if significant amounts of moisture will have found their way into the undamaged layup and you'll be taking out the damaged stuff anyway.

I'd be inclined to hack out the damaged grp and, if you're concerned about water in the layup, wash everything down with clean water and let it dry for a day or two before starting the repair.

One other thing, almost all chopped strand mat is designed for use with polyester resin, not epoxy. The binder that holds the strands in a mat disolves in the styrene in polyester resins, but not in epoxy, so it won't wet out properly. Use epoxy, 'cos it's a zilliion times stronger and sticks like the proverbial to a blanket. but use plain glass cloth or a special mat designed for epoxy.
 
G'day Nick,

I was going to say sorry to hear you damaged your mode of transport between shore and ship. But thinking about it you have had some good luck, after all you were in no danger when it decided to split, could have been another story at sea.

To help work out resin and cloth required for your repair (or anyone else for that matter) try the following:

I. estimate the size if the area to be treated and how many layers you plan to add and what thickness / weight cloth you plan to use.

2. For standard wax resins:
Total cloth/CSM weight times 3 = minimum resin required.
Based on the 3:1 ratio required, 1 part CSM to 3 parts resin.

3. For epoxy resin and cloth designed for use with epoxy:
Total cloth weight= total resin weight.
Based on 1:1 ratio required 1 part cloth to one part resin.

I would be applying epoxy resin, lighter and stronger in this application.

Tip: You can improve the finish and seal by adding one layer of fine roving's as a last layer, this will not require more resin if added to the still wet last layer, standard or epoxy resins will both work with this. The use of roving's helps to cover any untidy end strands that may want to stick up through the resin.

Never sand epoxy resin until AFTER you have washed it to remove curing residue.

Only mix small amounts of resin and only in wide shallow containers like 1 or 2 litre ice cream containers.

Use disposable latex gloves not plastic ones, they may dissolve and contaminate the work.

There is a lot more I could list but it's a long day and I have another starting 6 hours (Almost midnight here) let us all know if you need more info and how the project goes.

Avagoodweekend......
 
that's extremely helpful. so its csm and epoxy resin for 6 sq mtrs as i think i'll make it super strong and on the last layer after applying the resini'll add some of the fine finish cloth to make it smoother and when dried i'll paint over the repairs inside and out.
thanks to all for the help

regards nick
 
Nick - a small point -
there are at least 2 types of CSM.

Powder bound and emulsion bound and the rest.

Emulsion bound CSM is for polyester resin and epoxy resin has problems with it.
Powder bound CSM is designed for epoxy resin which is what you should be using for it's "stickability".

Make sure you get the right one or stick to cloth - the biaxial ones are expensive but very easy to use..
 
ive been out to measure what i need and i think its
5sqmtrs of 300gsm powder bound csm
4kgs of expoy resin
80grams of catalyst
3sq mtrs of fine tissue for finishing smooth
 
Check that list again, I dont think it should be 4kg of epoxy resin.
For every 100grms of cloth you should use 100grms of resin/hardner mix, so 300grm cloth will need 300grms of epoxy resin for every sq metre of it, so 5sq mtr will need 1.5kg of resin. which is an A pack of wessex resin/hardner, which comes in a boxed kit for around £35 from chandlers.
That is what is known as 105 resin and 205 hardner, which you should find easier to use as it is not too quick setting, and should allow you to use i litre milk containers with tops cut off to mix in.
Also remember 5 parts resin to 1 part hardner, as this is critical to have right.
And I am 99% sure that polyester gell coat can not be used over any epoxy repair, instead add a few coats of epoxy resin to the finnished sanded job as a waterproof barrier.
The chamfering of the damaged area should be done at 12:1 not 20:1 as earlier mentioned.
Good luck.
 
If this your first GRP repair may I suggest that you find a fiend who has some experience in it. Not because it is over complicated but it needs to be planned ahead.
All the glass should be cut to size before starting and all your tools ready at hand. Latex gloves are a must as epoxy is very aggresive, and some people are affected by just the fumes.
If you mix your resin in a container get it onto the job quickly as the mass of resin in a pot will cause an exotherm, wich is both dangerous and expensive.
As your boat is made of polyester why not repair with the same thing? . It's far cheaper and it will bond quite happily with properly prepared surfaces. Finally as no body has mentioned polyester needs the catalyst added at about 1.5%.
 
G'day Nick

Quote: <<so its csm and epoxy resin for 6 sq mtrs>>

If you plan to use an epoxy DO NOT use Chopped Strand Mat (CSM). the voids in this material will require a lot more resin to it out, use only cloth designed for use with an epoxy.

The use of CSM and epoxy will make the project very expensive and because you will have too much resin to cloth ratio, it will be weaker and heavier.

Avagoodweekend......
 
if i were to clean and abraid the areas properly then is polyester resin a good repair medium or does it have to be epoxy resin. the difference in price is quiet high
 
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.
 
You must log in or register to reply here.

Share this page

Top