How much epoxy at once?

[slawosz]

Hi,
I have some repairs, where I would have to lay around 10-12 mm of fiberglass with epoxy resin. Is it ok to do it at once, or should I do it in batches - for better curing etc?

 

[Bobc]

When it comes to epoxy, don't mix a big load in a pot at one time. It creates an exothermic reaction and will get very hot. Mix in a big pot and keep it shallow.

Best if you can do the whole job in one go and let it all cure together. Use a slow hardener and you will have a few hours working time with it (the fast hardener will only give you about 20 mins).

 

[Poey50]

Definitely good advice not to mix a big pot load at the same time because of the exothermic reaction as Bobc says. But fast hardener is more tolerant of lower temperatures. Just have several suitable containers ready to go and mix and use them serially.
205 Fast Hardener - WEST SYSTEM marine grade Epoxy

 

[Rappey]

Why use epoxy over polyester resin? I'm guessing your fibreglass in to fibreglass.
The binder in fibreglass mat does not dissolve with epoxy so makes it a little harder to go round curves and completely wet out, but it does get suspended in epoxy without any issues

 

[RJJ]

You can always add extra heat. Either gently warm the materials in a tepid saucepan before mixing, or wave a heat-gun once in place. Or both.

What you can't really do is take heat away once it goes exothermic. Made this mistake last year and sat watching about fifteen quid worth of resin go off smoking in a matter of seconds.

 

[lw395]

Apart from the exotherm issue, 10mm of layup can easily become a squidgy out of control mess.
Also if it does get hot, distortion may be an issue.

Personally I would do it in about 3 goes, getting the next batch mixed when the first batch has gone solid.
Or if the first batch isn't as pretty as you'd hoped, let it go hard, grind it to shape next day and go from there. This can happen when you're not working on a solid base, e.g. fixing a hole.

I would consider whether polyester is appropriate. The exotherm issue applies to polyester too, by the way.

 

[anoccasionalyachtsman]

4-500ml in a wide pot about as much as I can hand lay before it turns to gel. Alternatively, mix it in a pot and transfer to a baking sheet/oven tray if it's a warm workplace. Support it so that cooling air can get underneath.

 

[Ian_Edwards]

It's commonly understood that epoxy is a much better adhesive than polyester, if you use polyester , make sure that you have a good overlap to the sound material, a very good bevel angle or scarf, to give plenty of area for the polyester to bond to.
In terms of mixing, I agree with Iw395, it's probably better to let the initial lamination go solid, but not fully cured, that will give a better inter-lamination bond, and prevent the weight of the lamination cvausing the repair to sag

 

[SvenH]

As I understand it polyester does not bond very well to old cured polyester, this is why epoxy is a sensible choice.

"The binder in fibreglass mat does not dissolve with epoxy"
It used to be the mat was only to be used with polyester where indeed the binder would be disolved by the styreen in the polyester mix would loosen the fibers in the mat.

Now there is also mat available for use with epoxy.

 

[Yngmar]

I have some repairs, where I would have to lay around 10-12 mm of fiberglass with epoxy resin. Is it ok to do it at once, or should I do it in batches - for better curing etc?

If it's on a flat horizontal area (and supported beneath), you can lay it all up in one go. If it's vertical, angled or upside down, I would do it in batches, because if you put a lot of layers at once, they have to support quite a lot of weight and may sag or slide before curing and you get to grind it all out and start over again.

 

[coopec]

Apart from the exotherm issue, 10mm of layup can easily become a squidgy out of control mess.
Also if it does get hot, distortion may be an issue.

Personally I would do it in about 3 goes, getting the next batch mixed when the first batch has gone solid.
Or if the first batch isn't as pretty as you'd hoped, let it go hard, grind it to shape next day and go from there. This can happen when you're not working on a solid base, e.g. fixing a hole.

I would consider whether polyester is appropriate. The exotherm issue applies to polyester too, by the way.

I agree 100% that it should be done in three goes. 10mm is a pretty heavy lay-up.

Why use epoxy? Why not polyester?

 

[coopec]

As I understand it polyester does not bond very well to old cured polyester, this is why epoxy is a sensible choice.

"The binder in fibreglass mat does not dissolve with epoxy"
It used to be the mat was only to be used with polyester where indeed the binder would be disolved by the styreen in the polyester mix would loosen the fibers in the mat.

Now there is also mat available for use with epoxy.

Just recently I went to the expert at the fiberglass/resin suppliers (who has been in business for 40 years) for advice. I felt I knew everything but wanted to be sure as I was fiber-glassing in the SS anchoring points for the steering pulleys beside the quadrant and I wanted it to be strong.

The first thing he asked is "why are you using epoxy, you should be using polyester" . I thought, like you, I would get a better bond.

I am about to fiberglas in the anchor points for the autopilot and I will definitely be using polyester/CSM as CSM will wrap around the SS anchors much easier than WR.

 

[JumbleDuck]

Why use epoxy over polyester resin?

New polyester doesn't stick nearly as well as epoxy well to old, cured polyester. The cheap way round this is to use a layer of epoxy, then finish the layup with new polyester, but the extra material costs of epoxy are fairly trivial so it's easier to do epoxy all the way.

 

[JumbleDuck]

The exotherm issue applies to polyester too, by the way.

I once realised I had make a mistake in mixing 500ml of polyester in a single plastic pot when it started smoking. It's possible to mix large(r) quantities, but you need to arrange good heat dissipation or use a very slow setting resin or not much catalyst.

 

[thinwater]

If you use a fast setting resin, you can always lay the first 1/2, go do something else for 15 minutes, and then do the rest. It will still be green and cure as one unit, but the first part won't be moving around. I've done this many times.

For repair laminating it is generally a mistake to mix more than about 100 ml at one time. Larger batches are for big projects, after you know how fast you can work it. Remember, even on large projects, it takes some time to work the resin through the cloth.

I didn't see anyone mention using 1708 instead of mat. It's much stronger, builds fast, and goes limp like mat, even with epoxy, because there is no binder. Much better than roving. This is what a lot of cats go with.

 

[slawosz]

Thanks for all anwsers. I go with epoxy, as even being more expensive is often recognized for better adhesive, besides West System pumps make mixing very easy task. I don't need to do a lot as well.

 

[thinwater]

I gave up on the pumps years ago. A scale lasts longer, fits any container size, and allows any batch size.

 

[slawosz]

I gave up on the pumps years ago. A scale lasts longer, fits any container size, and allows any batch size.

I see. How about using cups with measurements?

 

[thinwater]

^^ Sure. But accuracy can be a problem in smaller batches.

Accurate ratios really are important to maximum strength. For example, I've had times when the mini-pumps had air bubbles or some other factor, dispencing volume was uncertain, and the result was substandard. The pumps get well-primed on a big job, but for little things, where you only need 1-2 strokes of each, they can be way off.

 

[slawosz]

The pumps get well-primed on a big job, but for little things, where you only need 1-2 strokes of each, they can be way off.

This would explain, why plywood primed with small batch was bit sticky for weeks. So weighting is actually the best? Normal kitchen electronic scale will do?