gelcoat and resin laminating: time between layers

[laika]

Every so often I bite off a little more than I can chew. Despite having read 3 different books on the subject I feel I still need to ask questions...

I'm trying to fill a hole in the cockpit where some speakers used to be. I'm a dab hand with gelcoat filler, but serious stuff is new to me. The previous owner had stuck circles of acrylic over the holes, but seeing as the headlining is down I thought I'd have a go at filling them.

I've ground down a bevelled edge on the inside (not 12:1 because there's not space, but it's not too bad), lightly sanded outside, acetoned everything, got a couple of bits of acrylic covered in releasing wax and screwed them outside the 'oles so I'm ready to go. My plan is paint a couple of layers of gelcoat on the wax (from the inside), then laminate csm and cloth behind it, then remove the acrylic and tidy up the outside. But...

* Do I need to add wax in styrene to the first (outer) coat of gelcoat (my primary book doesn't mention this), will the screwed down acrylic behind it allow it to harden without, or should I put gelcoat with wax in styrene over the outer layer later?
* How long between gelcoat coats (ie how hard for the first coat before applying the second)? Presumably this means 2 lots of mixing
* I'm aware that a little tackyness helps with the chemical bonding. If I don't get the whole job done in one day, which bit should I leave over night (ie if it's the end of the day, do I leave it at the second layer of gelcoat or do I do the first layers of resin and glass).

Sorry to ask what research should tell me: I've asked a couple of people who've given me two different methods and completely different to my books, so trying to crowdsource this

 

[prv]

Sounds like you're well prepared.

I would not add the wax to the gelcoat - being done effectively into a mould should be fine.

I've never used two layers of gel, though it probably will give a better result especially if polishing is needed (you don't want it so thin you sand right through it). Perhaps you could be slightly generous with the catalyst for the first one to get it set nice and quick so you can get on.

If you're prepared with all the bits cut out etc then the glass layers will go on quick - minutes, not hours. So I wouldn't worry about where to leave it, you'll be done long before that becomes an issue.

Pete

 

[VicS]

Sounds like you're well prepared.

I would not add the wax to the gelcoat -

I think PRV is right. Don't add the wax in styrene . It is only necessary when you are not laying up into a mould and the gel coat is the final outer layer applied. It tends to stay tacky in contact with the air. The wax prevents this.

If you add it to your gelcoat it will prevent the glass layup adhering to it wont it?

Apply the gel coat and allow that to cure before proceeding with the laminating. It should cure to a tacky surface onto which you can build up the layers of glass and resin.

One worry though ....... are you sure that acrylic is a suitable material for laying up onto ?? Or will the resin attack it ?

Plenty of advice on the resin suppliers websites also an old publication by Tyler Mouldings http://www.topcatsail.co.uk/GRP-REPAIR-MANUAL.pdf


Just about to do a similar job except that mine is a ( smaller) hole in the bottom of the hull and I will be using epoxy resin and biaxial cloth

 

[boguing]

It has been many years since I ran a small factory doing this stuff, but, this is what I know.

Gelcoat does not need wax if you are applying to a mould. Air cannot get to it, so it cures as it should. Typically, PVA would be used to create a breakable bond.

The sticky surface is now on the work side, which is deliberate, it lets resin bond to it more fully.

I'd certainly avoid direct contact with wax, in production it is used to make the mould as shiny as possible, but it's not something I'd want to contact the gelcoat.

If you are confident that the mould that you have made< one layer is enough. As Pete said< two if you are not.

Sadly, two batches neccy, but as soon as the first coat is tacky, on with the next.

You might get away with one mix and a hot air gun to accelerate the first coat, and the rest in the fridge.

Lay up starts as soon as gel is tacky.

This is all aimed at poly/vinylester resins.

 

[dovekie]

Yes, good advice so far I think. I've made several fittings (dorade vents, solid sprayhood, winch platforms, custom draining-board) using a female mould in this way. The other's advice is based on more experience than mine by the look of it about tackiness and lay up etc. But one mistake I have made is putting too thin a layer of gelcoat on, so that when I come to smooth and polish it I sand right through it. I would advise two or even three layers of gelcoat, and try and minimise the bubbles in the gelcoat as you mix it. Spreading it out moderately thinly on a board helps, before you brush it on the mould. Be carefull to fully wet the first layer of csm applied behind the gelcoat too.

 

[boguing]

dovekie.

The reasons that a yachtbuilder uses just one layer of gel are that:-

Gelcoat adds no strength, just weight and appearance.

If a thick layer were used it would be more prone to cracking as the boat flexed (gelcoat is very brittle).

They are not concerned about wearing through it by polishing in years to come.

Of course, it would cost more, but the above reasons suggest that it is not a good idea.

However, when repairing damage, and that lovely shiny mould is not available, two layers is wise if the area is nominally flat. If it is curved then more may be worthwhile. Just threat as a good surface to wet and dry to finish. Far easier than using a filler and finding all the bubbles as you finish it!

 

[laika]

are you sure that acrylic is a suitable material for laying up onto ?? Or will the resin attack it ?

Adlard Cole's book of hull and deck repairs which is currently my primary source suggested formica laminate or acrylic, so hopefully yes. Amazing how difficult it is to get something that people with houses and garages full of stuff just have laying about. I went into half the DIY and kitchen places in Fareham and Gosport today trying to get an offcut of something shiny.

My book suggested wax and then pva "just to be sure" for the releasing layer. The guy in the chandlery was quite adamant that they both did the same thing so I was wasting my money so I felt obliged to put the PVA back. From Boguin's post I'm guessing I should have stuck to my guns.

Also, my reading suggests that it's a bad plan to do more than 3 layers of layup at once. I'm guessing then that I shouldn't start unless I have the whole day to do 4 sessions of layering (2xgelcoat, 2x(3xresin/csm/fabric)).

Thanks for all the excellent advice: anyone got an opinion on the necessity of the PVA do post. The fewer things I can get away with buying the better, but not if I'm going to ruin the rest of the work

 

[William_H]

Yes you do need to leave a layer of gel coat to become stiff if not hard. If it is too soft your brush may pick up or penetrate the first gel coat so negating the value of a second coat or of gel coat at all. "gel" means hard enough to withstand the abuse of another layer of glass or more gel coat.
You will find then on a vertical surface that there is a limit to how much fibreglass and resin you can lay up before it starts to sag. This will be an indication it is time to let the resin go hard (or nearly so) before you try to pile more on. Yes you will need multiple mixes of resin allowing the previous mix to harden. The speed it hardens depending on ambient temp and amount of hardener. If you can afford to waste a bit of resin try double strength hardener but stop using the mix if it gets hot in the pot or starts to gel.
good luck olewill

 

[CET1]

If you don’t let the gelcoat gel to a 'hard' enough state then you can push the matting through. Two or more thin coats are best. Just be gentle when mixing and brushing the gelcoat on to avoid air bubbles. Also make sure at the edges of the patch that the subsequent layers of glass can adhere to glass and not gelcoat. Good luck

 

[VicS]

One worry though ....... are you sure that acrylic is a suitable material for laying up onto ?? Or will the resin attack it ?

Adlard Cole's book of hull and deck repairs which is currently my primary source suggested formica laminate or acrylic, so hopefully yes. Amazing how difficult it is to get something that people with houses and garages full of stuff just have laying about. I went into half the DIY and kitchen places in Fareham and Gosport today trying to get an offcut of something shiny.

OK fair enough.
It's just that acrylic is affected by many solvents I was worried that styrene might affect it too. Formica would have been ideal. Aluminium good also, even the smooth side of hardboard well waxed.
I could not find any information on the effect of styrene on acrylic except that its resistance to aromatic hydrocarbons in general is poor.

Id check for any effect of the resin on the acrylic before starting I think.

 

[nigelmercier]

anyone got an opinion on the necessity of the PVA do post...

In case anyone is confused, this is Polyvinyl Alcohol not Polyvinyl Acetate, which is PVA adhesive.

http://www.ecfibreglasssupplies.co.uk/c-821-pva-release-agent.aspx

Much better than wax alone, or even in place of it.

 

[VicS]

In case anyone is confused, this is Polyvinyl Alcohol not Polyvinyl Acetate, which is PVA adhesive.

http://www.ecfibreglasssupplies.co.uk/c-821-pva-release-agent.aspx

Much better than wax alone, or even in place of it.

I always though PVA (adhesive) was an odd thing to use as a mould release agent !!!!!

BTW Stuff I ordered yesterday late morning from East Coat Fibreglass arrived at breakfast time today via UK Mail

 

[nigelmercier]

I always though PVA (adhesive) was an odd thing to use as a mould release agent !!!!!

I'm not sure if you mean that, but I was certainly confused for a while. I figured that the "adhesive" wouldn't stick, and the film could be washed off. The mould release is similar to the solder mask used on PCBs, discovering that was when I realised that there are two different substances called "PVA".

You get your post at breakfast? I'm lucky if it arrives by lunch.

 

[TQA]

Are you using polyester resin or epoxy?

Be aware that polyester does not bond well to full cured old fibreglass.

 

[laika]

Are you using polyester resin or epoxy?

Polyester. It's not "load bearing" but there's a big area of gelcoat on the visible (cockpit) side. I understand ( from reading) that gelcoat forms only a mechanical rather than good chemical bond with epoxy. I'm hoping that a few layers of polyester of increasing size in the ground area should be strong enough. Planning on attacking the job when I have a whole day (early next week probably). Thanks for all advice: I'll report back on what kind of a disaster befalls me...

 

[VicS]

Polyester. It's not "load bearing" but there's a big area of gelcoat on the visible (cockpit) side. I understand ( from reading) that gelcoat forms only a mechanical rather than good chemical bond with epoxy. I'm hoping that a few layers of polyester of increasing size in the ground area should be strong enough. Planning on attacking the job when I have a whole day (early next week probably). Thanks for all advice: I'll report back on what kind of a disaster befalls me...

I think you are right. There is no chemical bond between polyester and epoxy, but there will be no chemical bond between your new and old polyester either.

Epoxy is often used for repairs because it a good adhesive and forms a strong mechanical bond with the existing polyester laminate. That is the reason I will be using epoxy on the job I have to do.

I think you are doing the right thing but I'd not bother about the 12:1 chamfer . I'd bevel the edges somewhat less than that but cover the whole repair with several layers of resin and glass on the inside over lapping well onto the existing. Assuming that is that the inside will be hidden and you are not attempting a repair invisible from both sides.

BTW

When you fill the ground out area , assuming you will be sticking to that idea, the first layer of glass should be the overall size of the chamfered area, then you add progressively smaller pieces, the final one being the original side of the hole.

 

[Bosun Higgs]

* Do I need to add wax in styrene to the first (outer) coat of gelcoat (my primary book doesn't mention this), will the screwed down acrylic behind it allow it to harden without, or should I put gelcoat with wax in styrene over the outer layer later? No you dont - the acrylic does the function of the wax in keeping air away from the gelcoat whilst it hardens
* How long between gelcoat coats (ie how hard for the first coat before applying the second)? Presumably this means 2 lots of mixing. Wait until the first coat gets tacky ( use a finger on the mixing pot) and then apply a second.
* I'm aware that a little tackyness helps with the chemical bonding. If I don't get the whole job done in one day, which bit should I leave over night (ie if it's the end of the day, do I leave it at the second layer of gelcoat or do I do the first layers of resin and glass). you'll easily get the whole jopb done in an hour never miond a day. Adjust the cataluyst ratio between 1% and 3% to get the speed you need. Plus you can laminate a few layers at once. When making some bits for my boat I laminated 4 layers of csm in one go

Sorry to ask what research should tell me: I've asked a couple of people who've given me two different methods and completely different to my books, so trying to crowdsource this

nice word crowdsourcing. But beware of doing that on here. Times many I have seen wrong info simply repeated by several people.

 

[dovekie]

Thank you. What an informative place the forum is.

dovekie.

The reasons that a yachtbuilder uses just one layer of gel are that:-

Gelcoat adds no strength, just weight and appearance.

If a thick layer were used it would be more prone to cracking as the boat flexed (gelcoat is very brittle).

They are not concerned about wearing through it by polishing in years to come.

Of course, it would cost more, but the above reasons suggest that it is not a good idea.

However, when repairing damage, and that lovely shiny mould is not available, two layers is wise if the area is nominally flat. If it is curved then more may be worthwhile. Just threat as a good surface to wet and dry to finish. Far easier than using a filler and finding all the bubbles as you finish it!

 

[Avocet]

Couple of things that haven't been mentioned yet:

Are these holes in a vertical or horizontal (upside-down) surface? The OP mentions a headlining being down. If you're trying to fill holes on the underside of a horizontal surface, it can be quite tricky (the wet mat will tend to fall off)! To a lesser extent, the same can apply if laminating on to a vertical surface.

I'd agree about a double gelcoat. I assume it's going to be pigmented? If so, don't overdo the pigment, it can retard the curing process significantly. It's amazing how little you need.

As has been said, the laminate can be applied as soon as the gelcoat has gone tacky. I used to work for a fibreglass car company and to be honest, they'd leave a gelled mould over a weekend if they got to the end of a shift, and put the laminate on the following week, so you can leave it quite a long time with no ill effects.

Subsequent layers of laminate can be put on as soon as the previous layer has started to solidify (tacky to the touch but no resin transferred on to the finger) BUT BEWARE that if you're putting a lot on at once, there can be very significant heat build-up. Presumably, this is a relatively thin panel though? Only a couple or three layers of laminate?

 

[laika]

Well goodness only knows what it's going to turn out like. Waiting for the third coat of gelcoat to harden. 3rd you say? Oh yes it's easy for a book to say "brush on the gelcoat" but gelcoat didn't really want to stick to the PVA-and-waxed surface so I ended up kind of stippling the first coat. Second coat I discovered that using a broad stick as a spatula seemed to work better than brushing. Third coat was easier and I filled the spots which were looking a bit thin. Worst case scenario I suppose I can grind down the mess I've made on the outside and fill it the other way round. Even if it ends up a disaster I suppose I'll know better next time. Better 'an book learnin'

Didn't try to colour match. I might regret it but thought I had enough to worry about already and I usually find with smaller repairs that once the surrounding area is sanded down and polished it looks relatively white and certainly after a few months of ageing. This is of course a "big" repair, but can't look more odd than the covering that was there before.

At least I've got a nice day for it