GRP hole repair, mixing resins

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m1nder

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Hi folks,

I need to do 2 things with a boat restoration.

1. Repair a hole in the hull
2. Coat the hull with epoxy and glass mat.

Now I am told that I cannot use gelcoat with epoxy so I need to do the hole repair with polyester resin. Is this correct?

Then, I need to epoxy the boat's hull. It's old and full of stress cracks so I want to do a nice new coat of epoxy and glass to give it new life. Will it be ok to epoxy resin to the cured hole repair which was done with polyester resin?

Many thanks
 
Repair the hole with epoxy and the correct glass. Then no need to use a gelcoat.

Started to look into this myself to repair a hole where the old toilet outlet is at present ( about 2¼" diameter)

I understand that you don't use the chopped strand mat that is used for polyester resin but a woven glass cloth without binder.

Thats going to be a big and expensive job sheathing the whole hull with epoxy and glass.
You dont think opening the cracks up filling the applying an epoxy coating will do ...too may cracks?

(One way round or the other, either polyester wont stick well to new epoxy or epoxy wont stick well to new polyester .. I forget which.)

Some useful info http://www.cfsnet.co.uk/acatalog/Epoxy_User_Manual.html
 
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Gel coat

Basically the boat is made of polyester resin. Gel coat is just the layer put on first on the mold so outside layer of resin only with colour. Polyester bonds really well to itself when laid up and for a period while it cures. After that polyester resin does not stick so well to it self or any other material compared to epoxy.
Epoxy tends to stick well to anything as a purely mechanical bond. Epoxy however fails in UV sunlight so is always painted.
So to fix a hole in the hull you can use polyester if it is a large hole and there is good mechanical bond. The classic case here is a hole for a transducer where you chamfer the hull so that the patch is bigger on the outside and bigger on the inside than the hole and the polyester chemically bonds where the 2 patches join in the middle. So even with no sticking the patch itself is locked in.
For smaller repairs epoxy is far better being a better adhesive.
I would suggest forget about gel coat unless you use polyester resin. gel coat will chemically bond to the polyester. however you will end up with layers of filler etc to get the required smooth surface. if you then are going to coat with epoxy as you would under water then just epoxy over the smoothed layers of repair.
If it is above water i suggest using a 2 pack polyurethane paint (sticks very well) and redo the whole area (boat) good luck olewill
 
The hole to repair is about 6-7". I want to use gelcoat so that I don't end up with a big translucent area. I plan to setup a flat waxed surface on the outside of the hull, paint gelcoat into that, and then lay glass / resin patches backwards into the hull to the same depth as the existing grp.

EDIT: Or lets ask a new question. Would it be ok to do the entire boat in polyester resin including the hold repair? (I hear that poly resin is not as good as epoxy) ?
 
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The truoble with using polyester is that it does not adhere awfully well to old cured polyester.

When laying up a new molding each layer is applied to the previous before it has fully cured. A chemical bond is then formed between successive layers.
When applied to an old fully cured surface only a mechanical bond is formed.
Epoxy will form a stronger mechanical bond than polyester. That is why I said to use it for the repair and its why I have been advised to use epoxy

Yes you can use epoxy over polyester but it may be necessary to allow some time for new polyester to cure fully

I'd recommend that you contact one of the major epoxy resin suppliers Eg Wessex resins, for authoritative advice.
 
m1nder said:
I want to use gelcoat so that I don't end up with a big translucent area. I plan to setup a flat waxed surface on the outside of the hull, paint gelcoat into that, and then lay glass / resin patches backwards into the hull to the same depth as the existing grp.
Click to expand...

Epoxy will give the highest bond strength and least water absorption, no question. No need for gel coat: if you don't want it translucent just pigment the epoxy (using a pigment specifically made for epoxy resin). Polyester gel coat in its native state is clear, just like epoxy. It only loses this translucence because pigment is usually added.

Glassing the entire hull is a very expensive monster job and you need to be very sure you need to do it. Trying to do it the way you suggest will almost guarantee large air-gaps. I'd strongly urge against it. There is a simpler way:

I've added a similar epoxy laminate on a large patch of my hull (about one tenth of the total underwater area). I started with a very thick high-build epoxy (ME-100 from AMC, the CopperCoat people). Applied this, waited until it was tacky, then rolled on woven rovings. They stick well. Then wait for the epoxy to harden (but not fully cure), which bonds the dry rovings to the hull suffciently for them to be worked on and to bear the weight of further resin. Then thoroughly wet out the rovings with ME-LV, a low viscosity solvent-free epoxy. This worked well without any tendency to allow air pockets, which can be a major problem with this sort of job.

Both ME-100 and ME-LV are much more moisture-tolerant than West epoxy and can be used in quite humid conditions. They're slow-curing, allowing plenty of time to work. I can PM you a spec sheet for both if you wish.

Suppose I should add that I have no association with AMC other than as a satisfied customer, although they once gave me a cup of coffee at a show.
 
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VicS said:
The truoble with using polyester is that it does not adhere awfully well to old cured polyester.
Click to expand...

To second that and expand with some advice I got from the polymers group I work with for a while ...

New polyester doesn't stick well to old polyester but does stick well - with one proviso - to new epoxy, which in turn sticks well to old polyester. So if cost is an issue, use a layer of epoxy on top of the old polyester and lay up with new polyester on top of that.

The proviso is that the epoxy surface may need to be abraded and dewaxed before the polyester is put on top.
 
m1nder said:
The hole to repair is about 6-7". I want to use gelcoat so that I don't end up with a big translucent area. I plan to setup a flat waxed surface on the outside of the hull, paint gelcoat into that, and then lay glass / resin patches backwards into the hull to the same depth as the existing grp.

EDIT: Or lets ask a new question. Would it be ok to do the entire boat in polyester resin including the hold repair? (I hear that poly resin is not as good as epoxy) ?
Click to expand...

It is unlikely that you can get a waxed surface that is perfectly contoured to the hull shape. You will almost certainly have to use filler to get the outside shape and surface perfect. hence the gelcoat idea will not work.
You need to shape the hole with a gradient or chamfer from the hole into the good surface. So you put firstly some pieces of cloth or woven rovings into the hole then a piece of cloth slightly large then another slightly larger each layer fitting into the chamfer or gradient. When you get the build up near to the surface either add a bit more glass or use filler so that you can sand down to perfectly level contour to the hull.
To achieve all this you need to put a temporary backing inside. A plastic bag of sand or similar can be wedged in giving a flat surface about in the middle of the f/g thickness onto which the first centre layers can be be laid. When the resin is hard or at least firm enough to hold its place and shape you take out the sand bag. Now do the same from the inside laying up the first layers in the middle onto the already laid f/g. If this second lay up is within a few days of the first lay up you will get really good chemical bond between the outside and inside lay up. You end up with a patch whose shape represents a YoYo ie the surfaces both on the inside and the outside of the hull are larger than the actual patch in the middle and the hole. It is mechanically locked in. Hence polyester would be OK to use. But epoxy is still far better.
As said just get some pigment to suit the resin you use. Put a little into all the resin you use that will fix the translucence. Finally fair on the outside with filler etc. good luck olewill

PS polyester as a coating of resin on all the hull will almost certainly peel off after a while due to lack of adhesion.
 
I am not sure if some of the replies have understood what I think you meant in your original question? Perhaps I am wrong as well, but here goes:

If the hole is in the hull above the waterline - in the topsides or perhaps the cockpit area, then you can achieve a fix using gel coat and epoxy. The GRP repairer did exactly this to a hole where a large log/speed fitting was previously mounted in the f'wd face of our cockpit and you would never know that the hole had been there when you look at it now.

Gel coat can be mixed with a special wax to make it set in air (normally it needs the absence of air to go off if I remember correctly). So you have a choice regarding fixing the hole.

If you can make a former that will fit exactly over the hole and you have access to the hole from the inside, you can grind back the edges all round and having carefully matched the gel coat colour and with the former coated with release agent and polished etc and taped and wedged over the hole, apply a couple of thick coats of gel to the inside. Apply the second coat as the first one is just going off, to ensure that there is a good chemical bond to the first coat. Then there is no reason not to use epoxy resin and mat to lay layers over the inside to repair the hole. All the usually caveats about making sure its wetted out properly and use the correct mat - for the epoxy. (Ask your supplier to make sure you get the right stuff)

Regarding the whole of the gel coat. I am not convinced about coating the whole of the hull/topsides in epoxy and matt. If the hull is still strong, but just has lots of crazing in the gel coat from being bashed about over the years, then it is possible to grind off the gel coat and trowel/paint on new gel and then long board and sand it all back with finer and finer grades and polish to end up with effectively a new gel coat. We had it done to our boat a few years ago and when done professionally, the result is spectacular. Several friends boats in our yard have been done as well and given skill and patience I suppose one could have a go at DIY job, but its a big task.

If you can only get at the hole from the outside, then there are other techniques, but before I go off into long explanations, perhaps you can give some more details?

Hope that this all helps. Regards John
 
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