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How to repair GEL coat?

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Old 03-16-2011, 09:39 AM
  #1
wildwillie3
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Default How to repair GEL coat?

Hi all, I have a pattern plane that I bought and some of the gel coat is missing from a minor crash. How can I repair or replace it? Or should Ijust do some body work to it. I cant add to much weight to it. I will try and take some pictures tonight so you can see what Iam talking about.

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Bill
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Old 03-16-2011, 03:37 PM
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Default RE: How to repair GEL coat?

If your fuse construction is epoxy glass then it does not have a gel coat surface. Gel coat is polyester based. What I would do if I was in your shoes is contact Comp ARF and order up some white touch up paint.
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Old 03-16-2011, 05:21 PM
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Default RE: How to repair GEL coat?

Piffel! I was hoping for more of an answer than that.
Walt
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Old 03-16-2011, 05:54 PM
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Default RE: How to repair GEL coat?

If the damage is minor, you could do as you said and fill it. I would sand the damaged area to remove any loose bits and fill with a feathering polyester glazing compound. You can then wet sand the area, prime and paint. If the damaged area cracked the fiberglass, you may need to sand the inside of the affected area, clean with denatured alcohol and reinforce the area with some 3 to 6 oz cloth and a good laminating epoxy system prior to the cosmetic repairs.

A few pictures of the affected area inside and out would help in deciding what may or may not be needed.
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Old 03-16-2011, 07:59 PM
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Default RE: How to repair GEL coat?

Finishing Epoxy. Let it dry, sand, paint to match. If the fracture is all the way through the part, follow Scorpion's advise on reinforcing on the inside of the part. If you don't want to paint, you can mix color pigment into the finishing epoxy.
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Old 03-17-2011, 03:07 AM
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Default RE: How to repair GEL coat?

Thanks for all of the advise so far I hope these pictures will give you a better idea of what I need to fix.

Bill

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Old 03-17-2011, 03:09 AM
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Default RE: How to repair GEL coat?

here's acouple more.

Bill
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Old 03-17-2011, 03:15 AM
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Default RE: How to repair GEL coat?

In the first two pictures the fiberglass does not feel or look like it has been damaged. It just feels weaker without the gell coat. if I just fill it with some polyester glazing compound will it remain flexable without cracking? What is Finishing epoxy

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Old 03-17-2011, 05:46 AM
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Default RE: How to repair GEL coat?

Bill - Finishing resin is probably a better way to put it. My bad, as the kids would say. Just look for the key word, Finishingon the label. Usually found along side of your standard Devcon 5min, 40min epoxies. Mix micro-baloons in to thicken. It will run, so lay the piece on its side for curring. Also, dome it up a little so you only have to do it once.It will remain tacy for about 24 hrs based on ambient tempature. After its dry to the touch, block sand and paint. Good luck with the repair. I have used this on several cowl repairs and it works good for me.
Local Hobby Store brands work just as good at about 1/2 to 3/4 the price.
http://www3.towerhobbies.com/cgi-bin...?&I=LXCX58&P=0
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Old 03-18-2011, 03:26 AM
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Default RE: How to repair GEL coat?

Thanks I will give it a try and keep you posted on the progress.

Bill
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Old 03-21-2011, 03:19 PM
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Default RE: How to repair GEL coat?

Any news?
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Old 04-07-2011, 02:51 AM
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Default RE: How to repair GEL coat?

I run a Glider repair business in Australia R/T AIRCRAFT SERVICES we specialize in composite repairs.>>

It looks like the basic matrix structure is damaged around the undercarriage mounts.>>

I can give you the information to repair your model or if there is a glider repair place close to where you live drop in and see them. We repair model aircraft and it usually works out cheaper for us to do it than >>

The owner to repair it themselves, mainly because we have all the materials in stock but we are also happy to sell the small quantities to them with the free advice and we also give them a Photostat copy of the relevant parts of the Manual of Repairs. You can also get the materials from Home build aircraft suppliers in small quantities.>>

As far as the exterior surface damage you need to sand the damaged paint work back being very careful not to damage the underlying glass epoxy structure. I would suggest you repaint the area with a polyurethane system we use DULUX highfill as the undercoat; I don’t know what is available in the USA. Go to a local motor vehicle repair place and they can supply you with all the paints and mix the colors to match. Hope this helps and if you need advice on structure repair I am quite happy to write them out for you. As far as gel coat repairs it’s a bugger to do on such small areas and the models we’ve repaired we’ve always used polyurethane Paint. With Gel coat you need to paint thin layers to stop pin holes and then spend hours sanding it back smooth with gradually finer wet and dry paper then polish it hoping you don’t go too far and have to start again. >>

All the best >>

Rachael >>

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Old 04-07-2011, 04:18 AM
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Default RE: How to repair GEL coat?

I have finally made some time to start making these repairs and it is going well so far. I will post some pictures as soon as I can.

Thanks
Bill
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Old 05-02-2011, 09:29 PM
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Default RE: How to repair GEL coat?

In the past I've used carbon fiber veil for an underlayment after sanding the top surface down just a little. I use it on the back side too for models. I've repaired some full boats and cars using a heavier carbonfiber cloth. It just weighs more for models.
the jell coat is not very strong itself but when it is stuck to the matrix material it adds quite a bit. It is really hard to get chopped fibers commonly used for filler to be strong enough to prevent future cracks. Also be sure to end any joining cracks with a drilled hole. If you don't it will crack down the road. Most of the time "down the road" doesn't happen in models as their life time is too short. haha
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Old 05-03-2011, 04:41 AM
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Default RE: How to repair GEL coat?

Chopped Fibers ? Jeess mate it's not a boat but a plane,there made from layers of cloth and epoxy.
We don't even class chopped strand mat as structural.
Ifyour giveing out advice Know what your talking about first.
Sorry mate but I've seen so much stupid information posted under this Forum it just got my goat so you've copped it.
REALY PEOPLE Before advising somebody don't if your not an expert on the matter.

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Old 05-03-2011, 08:45 AM
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Default RE: How to repair GEL coat?

I really think that the area around the landing gear needs "a couple layers of cloth" to repair it properly. Do this with the absolute minimum amount of epoxy as needed to stick each layer to the inside of the fuselage, blotting off as much of the excess of epoxy as possible, before it cures. Allow each layer you use to cure completely before you add the next layer.
For cosmetic repairs on the outside epoxy and micro-balloons make a nice filler and sands fairly easily, then just paint it up and your good to go.
To fix it right this is a bit of a project, it's going to requiring a lot of sanding and time on your part to fix it right.
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Old 05-03-2011, 09:01 AM
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Default RE: How to repair GEL coat?


Quote:
ORIGINAL: iron eagel
.....Allow each layer you use to cure completely before you add the next layer.
Why would you do this? Wouldn't molecular linking between the layers be much stronger than just a mechanical bond between them?



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Old 05-03-2011, 10:26 AM
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Default RE: How to repair GEL coat?

It is my understanding that it will adversely affect the curing of the next layer, I could be wrong I recall seeing something that said that it was an issue.
Perhaps someone who works on a regular basis with composites could correct me if I am wrong.
I'd think as you do that a molecular bond would be stronger myself, but like I said I read that it is an issue with the "cure".
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Old 05-03-2011, 12:06 PM
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Default RE: How to repair GEL coat?

YEP! Your wrong Molecularcross linking is way stronger than mechanical bonding which is what you get when you put wet on dry.
The calculation for joining dry joints is the strength of the cloth divided by the strength of the Epoxy.
E type Glass cloth is 55000 Lbs per square inch and epoxy 10,000 Lbs per square inch. Hence if you repair a crack you allow 3/8" per layerin a tapered scarf joint
if 2 layers taper the edges 3/4" 1st layer goes out the whole3/4" next layer is cut 3/8" smaller wet on wet.In an ideal layup the epoxy should weight the same as the cloth used."Ideal world"
temprature plays a part also, after solidification the layup should be held at a temp. of about 50c for 12 hours.
Gets involved but a model hasn't got a person in it so a bit of fudging will not hurt to much.
ALLTHEBEST
Rachael
PS If I get the time I may post a set of instructions on how to repair differant types of damage in area's Skins Gear Mounts Bulkhead attachment etc.
Quote:
ORIGINAL: iron eagel

It is my understanding that it will adversely affect the curing of the next layer, I could be wrong I recall seeing something that said that it was an issue.
Perhaps someone who works on a regular basis with composites could correct me if I am wrong.
I'd think as you do that a molecular bond would be stronger myself, but like I said I read that it is an issue with the "cure".
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Old 05-03-2011, 12:29 PM
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Default RE: How to repair GEL coat?


Quote:
ORIGINAL: iron eagel

It is my understanding that it will adversely affect the curing of the next layer, I could be wrong I recall seeing something that said that it was an issue.
Perhaps someone who works on a regular basis with composites could correct me if I am wrong.
I'd think as you do that a molecular bond would be stronger myself, but like I said I read that it is an issue with the "cure".
On large thick layups with resins that generate a lot heat while curing (polyesters) there can be issues. The building heat accelerates the cure too quickly resulting in a lot of molcular suspension and poor linking. On thin layups (less than 3/16") with polyesters there isn't a problem unless the resin contents are really high. When you use epoxies you can do layups that are 1/4" thick without problems (20 layers of 6 ounce glass). Most RC repairs and layups can done in one curing session because they are quite thin. Repairs should always be done with epoxy regardless of what was used on the part because of it superior adhesion properties.

Mrs R, wouldn't a glass layup that has a 50:50 ratio by weight be very resin rich? That translates to a 30 (fiber):70 (resin) ratio by volume which is pretty poor. We typically get 65:35 ratio by weight (44:56 by volume) with e-glass.

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Old 05-04-2011, 10:26 AM
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Default RE: How to repair GEL coat?

Good information!
I have only done fairly thick layups so far and have not done a model using that much composite material yet. Most of my use of composite materials have been for repairs not for actual construction and it has held up well. I have used carbon fiber veil and it has worked out well for what I was using it for. I am looking at doing a model in the near future that will be mainly composite construction and am glad to have input on the process.
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Old 05-04-2011, 05:00 PM
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Default RE: How to repair GEL coat?

Why wouldn't chopped fibers be considered structural??? What else would use be using them for? I'm thinking that this would be used for a fillet between, say, a bulkhead and fuse sheeting.

Bob
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Old 05-05-2011, 04:32 AM
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Default RE: How to repair GEL coat?

This looks like a Comp Arf or alike, with that said, that is most likely not gel coat, it is a paint in the mold process. paint or gel coat that thin will not add any strength or regidity to the composite substrate, so make your repairs to the inside of the fuse by performing a layup extending about .5" larger then the obvious damaged area. you will only need one ply of 3 to 6 oz. cloth and believe it or not, med CA with no kicker and your finger does as good a job as as anything els you can do, or A good epoxy resin to use is safety epoxy 10 from Aircraft Spruce & Specialty. This stuff works very well with Polyester or epoxy subtrates. I have done this on many airplanes for myself and others for a long time. On the outside, you will just need to feather edge sand, prime, and I would base coat and then blend a little clearcoat into the existing fuse.

Bob
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Old 05-05-2011, 06:33 AM
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Default RE: How to repair GEL coat?

My post stated that the weight "NOTVOLUME" of resin should equal to the weight of the cloth in these thin lightweight layups.
This is in an Ideal World but on repairs of model aircraft even we can not achieve and that is a layer of peelply with an absorbent under vaccum.

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Old 05-05-2011, 07:02 AM
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Default RE: How to repair GEL coat?


Quote:
ORIGINAL: Mrs R

My post stated that the weight "NOTVOLUME" of resin should equal to the weight of the cloth in these thin lightweight layups.
This is in an Ideal World but on repairs of model aircraft even we can not achieve and that is a layer of peelply with an absorbent under vaccum.

KNDREGARDS
Rachael
I understand that. My point is that 50:50 ratio by weight with e-glass is a very poor resin to fiber ratio. That's a lot of resin. The density of glass is 2.55 g/cc and the density of most resins is around 1.1 g/cc. A 50:50 ratio by weight translates to the 30:70 by volume. Only 30% of the layup would be fiber. You must be using really bulky fabrics with a lot of crimp. In a less than ideal world you can get a 60:40 ratio (fiber is first) without a peel-ply or a vacuum bag (with the right fabric choice).

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