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Fairing a polyester hull

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Paramedic View Drop Down
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    Posted: 06 Oct 15 at 8:29am
at 5mm deep you'd be ok to fill with gelcoat, but i wouldn't if it was too much deeper. It's inside my personal limit for filling a crease though, it's presumably not a flat 5mm deep and the hull is presumably pretty stiff at that point.

The trouble with gelcoat is it's not very flexible, but in this case as long as the prep of the dent is OK i'm pretty confident gelcoat will be fine.


Edited by Paramedic - 06 Oct 15 at 8:33am
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ChrisB14 View Drop Down
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Post Options Post Options   Quote ChrisB14 Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 15 at 5:52pm
It is indeed a matter of a single dent left by a previous owners wrong trolley cradle. about 5 mm deep, 200 mm long and 80 mm wide. At that depth I am assuming some of the foam has been compressed. 

I am not planning on getting anywhere near the foam and everything will be done with basic hand tools (sanding block, fairing board, batten). It is a relatively low value boat that will require a new paint job at some point, so I do not intend to just cover a patch with paint.

I have no idea why anyone would consider using car repair putty or anything along those lines. Seems odd. Even Plastic Padding fillers strike me as suboptimal. At the same time my understanding is that using epoxy is a bad idea when a gelcoat finish of the repair is the aim.

By the sounds of it any general use of marine polyester from a composite supplier (cfs, east coast resin, etc.) should be fine?
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 15 at 11:28am
To me the size of the job doesnt really matter whether you are fairing up a complete hull or doing a small local refairing I'd rather use the best materials. However the reality is that for small jobs most people go for a quick polyester filler and then go straight on with top coat. Often the oroblem is that its difficult for retail customers to get small amounts of the right materials so its a trip down to halfords for the P38. Sometimes its the only practical option.
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 15 at 9:41am
I think we might be picturing two different jobs here, oats, one doing small dents, the other the whole hull. Either way, mind, the foam should not be on view, as you say! I had visions of filler on top of shiny gelcoat, though, which really wasn't going to work.
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 15 at 8:15am
Chris,
You don't want to be anywhere near the foam when you are preparing the surface. If you have done that you have a real problem. You need to only key the surface, and a good filler will stick to this. If you start to see the first fibres of the surface plies stop. Your hull will have a series of hollows where the hull shell lamiante has shrunk onto the support structure. You can either fill the hollows or try to remove the bumps to get a fair hull. You want to do the former not the latter. Using a straight edge and an old sail batten find the hollows, mark them and see how low they are. This tells you where and how much filler to apply. You will need a few goes on the filler to get it fair. ( using a fairing board not a power sander) Working hard at this stage will make the final stage easier. A high build epoxy primer is then applied faired back, sometimes a few coats are needed. Finally a 2K paint system. With a job like this you can miss out stages or use other materials to save time and or money but this is how it should be done IMO!
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Rupert View Drop Down
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 15 at 8:41pm
Grit that coarse should give a good enough key. Gelcoat tends to be slightly waxy, and stuff can simply peel off, especially if you have a feather edge.
Firefly 2324, Lightning 130, Puffin 229, Topper 44496, Minisail 3131, Minisail 3446
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Post Options Post Options   Quote ChrisB14 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 15 at 8:33pm
I am planning to grind into the dent with 60 or 80 grit paper. If I can get away without grinding into the foam, I am happy. I wasn't aware that gelcoat was such a bad surface to bind to.
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 15 at 8:30pm
Are you planning simply to fill over the top of the gelcoat? If so, I'd say that more important than the type of resin is the preparation of the surface. Nothing really likes sticking to gelcoat.
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Post Options Post Options   Quote ChrisB14 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 15 at 7:16pm
Thanks for the helpful replies!

We might want to add a layer of gelcoat, so would shy away from epoxy. 

Paramedic: any particular brand or make of polyester that you would recommend?
B14 GBR 748 Bullet B
In build: Farr 3.7 GBR 410 (both sail number and the current number of loose parts)
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Paramedic View Drop Down
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Post Options Post Options   Quote Paramedic Quote  Post ReplyReply Direct Link To This Post Posted: 01 Oct 15 at 6:45pm
This depends on what you plan to do afterwards. If you paint the hull it doesn't really matter too much what you fill the dents with.

If you are planning to do gelcoat repairs then i wouldn't use epoxy as unless conditions are ideal you will have trouble getting the gelcoat to stick properly. The two systems are inherently incompatible and unless the epoxy is super well cured and was mixed at 100% the correct ratio you will have problems. Cooking the repair sorts most problems out, but may cause other issues on a polyester hull.

Personally i'd use polyester on a polyester boat. But i'd use good quality resin, glass bubbles and silica to attain the desired consistancy. Polyester stopper or car filler is not the way forward. 

Polyester systems get a bad press, but if it's what the boat is built out of theres not a great deal of point in using anything better. Just use good quality products from a composites supplier, not P38 or David's Fastglass.


Edited by Paramedic - 01 Oct 15 at 6:48pm
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