Fibre glass repair..thoughts

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Matata

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I want to repair an area where a wooden ply horizontal platform meets the ascending side of the hull. The length is approx 2 meter long , up to 2cm deep and 2cm wide . Its like an irregular trough/channel. To fillet it in, after good preparation, i was going to make up a filled solution of epoxy and fine milled glass fibres so that it was like heavy mayonnaise /light peanut butter consistency and put it in with a cake makers piping bag. Then trowel / smooth over it. Once it was starting to go off but not set I was going to put a wide bandage of glass over it, say 5cm either side of the trench....now my question! Is mild glass fibre easy to work with? will it blend into a flowable consistency? Where can I get them. What size fibres would you recommend> Ta nik
 
Is it necessary to fill it? Could you not just bridge the gap with fg tape? Alternatively, fill the gap with a balsa strip, set in with resin, sand to shape and apply tape and resin over the top.
 
Seems an expensive way of doing it and you might have trouble with the heat generated when the epoxy starts to cure. I would fill the void with expanding foam then apply you glass fibre tape over it
 
Milled glass fibre mixed with epoxy forms fairly heavy and saggy paste. Your proposed way of gluing the cross menber in to start with and then using a good fillet to give the laminate a fair radius of joint is a sound approach. I would be inclined to use a normal bonding/filleting powder rather than milled GRP, this is much lighter and does not sag as much as a GRP milled paste and still has the required strength. Good luck with that and remember to have your laminate layers in decreasing or increasing sizes so as not to create a hard spot in the hull.
 
Hi, Parsifal's way of doing it sounds good. If you do need a higher strength job, fill the groove with your epoxy mix in a few layers so the heat can dissipate. West Marine do fillers for their epoxy which are specific to different applications. They are not expensive. Then overlapping layers of tape on top as previously suggested.
 
With odd shapes and curves to go round I always find it easier to wet out a maximum of 600gm, 3 x 200 gm of CSM on a board/sheet of plastic away from the job, then apply it to the work. Get it ready then carry the board/plastic to the job. You want to get increasing thickness from the outside in, as stated above, but this can be achieved by cutting all the strips the same width and offsetting by an inch, say. That way it is all wetted out and filling the corner becomes unnecessary, unless this is vital structural work.
 
philwebb said:
West Marine do fillers for their epoxy which are specific to different applications. They are not expensive.
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They are very expensive, especially if bought from marine outlets.
Compare microfibres (which is what you'd use for the highest strength fillet):
West, £12 for 150g (£80 per kg): https://www.amazon.co.uk/West-System-403-Microfibres-150g/dp/B00G5Q8O10
SP, £15 for 500g (£30 per kg): http://www.mbfg.co.uk/epoxy-resins/microfibres-filler.html

I suspect this is precisely why Force4, for instance, stock the SP version. West is very convenient, mainly because it has excellent distribution in the marine trade and excellent 'how to' information. But it's a rip-off.
 
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You don't say where you are or when you intend to do this job. If you are anywhere in the UK or northern Europe, read the instructions for the resin very carefully if you intend to do it any time soon - the weather even on the south coast at the moment is consistently below the minimum working temperature for most epoxies. I have a significant job to do and have put it off till next spring.
 
An applications engineer from Wessex Resins Ltd (WEST's UK suppliers) told me that, as a rough guide, if where you are working is not warm enough for you to wear a T-shirt, it's too cold for epoxying.
 
Thanks all for very good help and advice, We were going to use west 205 harder which can be worked down to 5 degrees. We'll also use heat lamps. I think we'll do all the prep before Christmas . Then leave it till spring. There's plenty of other jobs to keep us entertained! Ta Nik
 
Epoxy will attach better to the wood and the old hull. So is recommended. However polyester resin may suffice and is easier to manage hardening times in various temperatures. The mixed chopped strands in resin are I think usually polyester. They work OK but are not epoxy. Using chopped strands of glass in the epoxy will give more strength. However fillers like micro balloons are easier to work with.
One trick you might consider is to use "twill weave" glass cloth where a warp might miss a weft every 2 or 3 threads making the cloth more flexible in all directions. An alternative if you can find it is to buy glass cloth woven on the bias. ie threads at 45 degrees to the length of tape. Or lastly cut tape from a square of cloth diagonally. These options mean that the tape can be pressed deeper into the corner so using less filler and giving more strength to the fillet. However of course you need the glass making a larger fillet for more strength. good luck olewill
 
It is a myth that epoxy will bond to cured polyester better than polyester will.

Polyester to uncured polyester results in a chemical bond and a mechanical bond.

Polyester to cured polyester results in a mechanical bond only, epoxy does exactly the same.

Using epoxy with GSM is also a big no-no as the binding agent does not disolve in epoxy resin. As is typical with this forum a simple GRP repair, requiring simple materials and technique is being made a meal of. Filling the groove with chopped GSM and polyester resin, topped with some layers of GSM and resin, rolled over with a consolidating roller will make a perfectly good job of this repair, cosmetically and structurally. It will also be far less susceptible to temperature issues. The P40 product i mentioned earlier is ready mixed resin and chopped strand, stiff enough to do this job without sagging.
 
PaulRainbow said:
It is a myth that epoxy will bond to cured polyester better than polyester will.
Polyester to cured polyester results in a mechanical bond only, epoxy does exactly the same.

I disagree as do most experts. Epoxy does a better job of bonding to cured (old) polyester, wood metal or most other things. This is because it has less shrinkage on curing and better inherent ability to enter and cling to imperfections in the base material. As an example epoxy has been used successfully to bond aluminium in crack repairs in aircraft wings or to bond gyrocopter rotor blades. That does not mean polyester will not stick just not as well, although in my experience long term it is hopeless on wood. olewill
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William_H said:
PaulRainbow said:
It is a myth that epoxy will bond to cured polyester better than polyester will.
Polyester to cured polyester results in a mechanical bond only, epoxy does exactly the same.

I disagree as do most experts. Epoxy does a better job of bonding to cured (old) polyester, wood metal or most other things. This is because it has less shrinkage on curing and better inherent ability to enter and cling to imperfections in the base material. As an example epoxy has been used successfully to bond aluminium in crack repairs in aircraft wings or to bond gyrocopter rotor blades. That does not mean polyester will not stick just not as well, although in my experience long term it is hopeless on wood. olewill
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My 35 year old Westerly disagrees :)
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Regarding temperatures I recently epoxied the base of my anchor locker in about 4 degree weather......put a fan heater on the foredeck with the locker lid propped up so it reflected the hot air (medium heat) into the locker.....cured in a couple of hours
 
PaulRainbow said:
It is a myth that epoxy will bond to cured polyester better than polyester will.

Polyester to uncured polyester results in a chemical bond and a mechanical bond.

Polyester to cured polyester results in a mechanical bond only, epoxy does exactly the same.

Using epoxy with GSM is also a big no-no as the binding agent does not disolve in epoxy resin.
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I believe the "accepted wisdom" (i.e. I Am Not A Chemist so could easily be wrong but what I have read in my fibreglassing tomes) is that as you say, polyester to uncured polyester forms a chemical bond but that epoxy to cured polyester forms a better mechanical bond than polyester to cured polyester.

Accepted wisdom also says that polyester-bound CSM (i.e. the normal stuff) shouldn't be used with epoxy because, as you say, the binding agent doesn't dissolve as it does in polyester and accepted wisdom is to use (slightly harder to find) powder bound CSM instead.

However...see my post in a thread from a couple of years ago about how the "pros" in the yard I was in used regular CSM with epoxy and why "accepted wisdom" is not necessarily a hard and fast rule (summary: there's a fair bit of wiggle room in this boat work malarkey)
http://www.ybw.com/forums/showthread.php?422478-Can-Chopped-Strand-Mat-(CSM)-be-used-with-EPOXY&p=5174063#post5174063

The contention that "polyester will form a mechanical bond with cured polyester good enough for most use cases" is one thing, but do you have any empirical evidence that the superiority of epoxy's mechanical bond is a myth?
 
laika said:
The contention that "polyester will form a mechanical bond with cured polyester good enough for most use cases" is one thing, but do you have any empirical evidence that the superiority of epoxy's mechanical bond is a myth?
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No, poor choice of wording on my part.

It is a myth that polyester to cured polyester does not provide an adequate repair and epoxy must be used.

Boats built using polyester were not all built in one go before anything had fully cured. Topsides are mechanically bonded to hulls. All manner of bulkheads and interior joinery is also mechanically bonded to fully cured polyester.
 
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