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Repair of Fiberglass cowls

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Repair of Fiberglass cowls

Old 03-30-2020, 06:07 PM
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Default Repair of Fiberglass cowls

I'm repairing cracks and breaks in a fiberglass cowl for a 60cc engine plane.
I have heard I can apply spray on adhesive to the inside of the cracks/breaks to hold the glass cloth down tight before applying the epoxy filler.
Will the spray adhesive affect/degrade the epoxy strength, etc.??
I know it makes the process much easier and less messy.... Name:  icon-status-online.png
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Old 03-30-2020, 06:40 PM
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If you can post some pictures of your damaged cowl I an sure I can suggest a few things to save you some time and effort. I am currently doing some work on a cowl that was somewhat overcut.




Old 03-30-2020, 06:49 PM
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great... I'll try to get some pics out tomorrow.
Thanks, Don

Last edited by dksnyder; 03-31-2020 at 12:59 PM.
Old 03-31-2020, 01:17 PM
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Default Pics of my cowl damage



I'll have to stabilize this crack
and rebuild the cowl inlet lip.
Any suggestions?
Old 03-31-2020, 04:04 PM
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That is actually going to be much easier to fix then what my explanation would indicate. First the front of the cowl, cut away the damaged area to give yourself a clean edge. Then take a 1/4" or 3/8" thick balsa peice and cut to match the shape of the desired opening with a little extra on the bottom. This will be used as your male mold. Once you have the correct shape, cover the outside surface with Monokote and apply a couple coats of wax. Tack glue the peice to the inside of the cowl. Using some epoxy laminating or finishing resin, lay a few layers of 1.4oz fiberglass cloth on the filler peice. The lightweight cloth should go around the corner of the intake lip well but make sure you give it a little radius. Once the new fiberglass cures, remove the balsa peice, sand the inside of the repair and apply a couple more layers of cloth.

For the other crack, wick some thin CA into the crack then sand the exterior past the gel coat about 1" each side of the crack. Apply a couple layers of 1.4oz cloth and epoxy resin. Let that cure and then sand the interior where the crack is and apply a couple layers there too. Feather sand the edges of the repairs on the exterior, prime and paint.
Old 03-31-2020, 07:14 PM
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Great... Thanks!!
Sounds doable......

The balsa male mold piece, with a little extra on the bottom (for attachment purpose?) matches the shape of the missing section of cowl?
Tack glue the covered piece... covered with MC... glue with epoxy on the extra bottom piece? epoxy on top of the MC?? does the MC cover the extra bottom piece?

I'll give it a try....

The crack should be easy... I hope!

Many thanks!
Don in Connecticut
Old 03-31-2020, 07:37 PM
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Don, pretty much yes to all your questions except tack glue the male mold peice with CA. Epoxy won't stick to waxed Monokote. You can of course trim the covering away from the area getting glued. Feel free to ask questions as you go through it, I check RCU everyday. Post pictures between steps if you can.
Old 03-31-2020, 08:04 PM
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Originally Posted by dksnyder
I'm repairing cracks and breaks in a fiberglass cowl for a 60cc engine plane.
I have heard I can apply spray on adhesive to the inside of the cracks/breaks to hold the glass cloth down tight before applying the epoxy filler.
Will the spray adhesive affect/degrade the epoxy strength, etc.??
I know it makes the process much easier and less messy.... Attachment 2266855 Attachment 2266856 Attachment 2266857
According to the way you presented your question, I would be inclined to think that you're not totally in dark when it comes to fiberglass.

When it comes to specific methods of repair, there are probably as many possible techniques as there are people who will tell you how to do it and they will all be easier and more fun to do than it would be to try to explain them. My suggestion would be to play around with the materials you plan to use on scrap in order to practice and gain familiarity with how the stuff handles and behaves.

As far as your idea of using spray adhesive to position fabric, let's just say that this indicates that you're plenty smart enough to navigate through this kind of repair. What I will suggest to you however is that you use only the lightest mist of adhesive which will hold the cloth in place perfectly. Using an absolute minimum of spray adhesive will not only make it easier to handle and position the fabric, but will also not inerfere in any way with the resin that you use with the cloth. It's my own preferred method for positioning structural fiberglass and I've never experienced any problems at all.
Old 04-01-2020, 03:41 PM
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Thanks again....
I probably won't start the process for a few days.
I'll keep you posted (with all the swear words deleted).
Old 04-02-2020, 09:39 AM
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I'm having trouble sending a picture of what I plan to do.
Is there some other way to communicate?
PM or E-Mail?
Old 04-03-2020, 07:15 AM
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A few questions on Cowl Repair. I'm concerned about how to secure the balsa filler piece for sanding, and how to resecure it after applying monocote.
Old 04-03-2020, 02:31 PM
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Don, the area that you being cut out needs to stay, that will be the area that gets tack glued to the inside of the cowl. It may require sanding to match the inside contour of the cowl.
Old 04-03-2020, 03:03 PM
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I don't quite understand that....
I cut the broken cowl away to a clean edge.
I measured the shape of the "missing area" of the cowl.
I will make a balsa piece to fit "exactly" in the removed area.
I then need to attach it to the inside of the cowl so I can sand it to the proper cowl contour.
Then I remove the balsa piece and cover it with monocote.
I put the covered piece back in place, and some how reattach it so I can apply FG cloth over the male, MC covered male form
Then I apply FG cloth over the form, and feather sand.

You say use CA to reattach the MC covered balsa form. Do I actually firmly glue the form back in place?
It needs to be removed after the FG outside application....I can't actually glue the MC piece to the cowl, can I?
Don't I want to support the "plug" from the inside, so it is easily removed after the FG application?
That is the part that me somewhat confused....I have to glue it in to hold it while I shape the balsa plug.... then It must be removable so I can add the monocote.... then it must be reattached firmly enough to support the FG cloth applications.

I was thinking that I might glue a balsa strip to the back of the balsa plug that can be glued to hold it for shaping,
and be more easily removed to apply the MC, and reattached, and then removed...

What do you think...
Old 04-03-2020, 03:57 PM
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Don, you make the balsa peice to fit behind the missing area. It will overlap the cowl material but from the inside. Then you lay new fiberglass on top of the balsa peice after it is sealed with Monokote and waxed.
Old 04-05-2020, 03:58 PM
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OK.... balsa plug installed, ready for sanding to shape. I used "Bob Smith" Foam-Cure to attach plug. It seems to be holding well, and should be easy to remove when reglueing with monocote applied.
Old 04-07-2020, 04:18 PM
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Balsa plug sanded

Covered in Ultracote (not monocote... still OK?)
Old 04-08-2020, 03:11 PM
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Looks good, wax up the plug but sand the white gel coat off around the damaged area before reattaching the plug. Your new glass won't bond to the gel coat very well.
Old 04-08-2020, 04:38 PM
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Yeah, that's a good point.... I'll sand all around real well.

Today I did a test of wax over a test piece of ultracote.
I used Meguiar's Super Shine high gloss "protectant"... a car wax/shine product
and Future Premium Floor Finish... acrylic protection
Both are waxy, shine products.

However, the covering didn't exactly "pop off" the fiberglass/epoxy surface...
It came off OK when I was able to grab a corner of covering and peel it off...
I'm kinda worried about getting the plug off/away from the fiberglass patch.
What do you use as a wax?
Old 04-08-2020, 04:54 PM
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I use Partall #2 but any good carnuba wax will work. I use the Partall in conjunction with PVA when molding parts so it is readily available to me. When doing simple things over a male mold with covering, I use wax only.
Old 04-13-2020, 04:02 PM
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Well, I got the patch covered in 2 layers of FG cloth.
That part went well....
Getting the balsa plug out was another story....
I finally dug out the balsa plug and the results were not too bad. Not professional, but OK for my use.
I don't think I'll ever get the patch totally smoothed out.... damn, that epoxy is tough to sand!!
I'll add a couple more layers of FG on the inside for strength..

Now I starting to think about the repainting process....
What kind of paint process do you use.??
I'll try to get something like Bondo to help smooth things out, then add filler and primer.
I want to match the nice, shiny, glossy white paint on the original.
Think something as simple as Rustolium (or something like that) spray cans will work?
Old 04-13-2020, 04:58 PM
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Don't use Bondo, it's polyester based and does not stick to epoxy well. I would take some of your epoxy that you used to make the patch and mix some micro balloons in to make a paste and use that as a filler. On a gasser you can use Rustoleum products. The epoxy appliance White is a great match for Monokote and Ultracote and is gasoline resistant. I have used it several times. I like to warm up the can under hot tap water prior to spraying. It atomizes the paint a bit better and the paint flows better on the surface.
Old 04-13-2020, 05:10 PM
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Originally Posted by dksnyder

Well, I got the patch covered in 2 layers of FG cloth.
That part went well....
Getting the balsa plug out was another story....
I finally dug out the balsa plug and the results were not too bad. Not professional, but OK for my use.
I don't think I'll ever get the patch totally smoothed out.... damn, that epoxy is tough to sand!!
I'll add a couple more layers of FG on the inside for strength..

Now I starting to think about the repainting process....
What kind of paint process do you use.??
I'll try to get something like Bondo to help smooth things out, then add filler and primer.
I want to match the nice, shiny, glossy white paint on the original.
Think something as simple as Rustolium (or something like that) spray cans will work?
That "something like Bondo" should be polyester glazing/spot putty. It takes the place of the red putty you used to see smeared over auto body repairs. My personal favorite of these products is SpotLite by Evercoat. If you Google it, you'll find it's available online from various suppliers.

SpotLite is my favorite because it sands so well and so easily that it's almost fun to do and the results are definitely worth it!!!

It's a professional product so it isn't likely that you'll find it at your local auto parts store.

As a last resort you CAN use Bondo, but it's MUCH harder to achieve comparable results!

Last edited by airsteve172; 04-13-2020 at 05:15 PM.
Old 04-13-2020, 06:49 PM
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Unless I'm mistaken, it seems like you're looking for a really smooth slick finish on the cowl. So here's a suggestion for paint that might be a little different.

Of course the surface must be made as perfect as you can get it using lots of coats of primer and lots and lots of sanding between coats with appropriate grit sandpaper.

Assuming you have the surface prepared to the degree you want, so what kind of paint? The usual approach is some kind of mediocre hardware store spray can paint and if all you care about is color, that'll be fine, but if the quality of finish matters to you (as it doesn't seem to matter to most modelers) then you basically have 2 choices. Either go high end automotive 2k urethane or a cheaper paint called lacquer.

Lacquer is relatively easy to use and dries quickly. It's sometimes available in the spray paint department at a hardware store, but not always. Auto parts stores usually have auto touch up spray paint like Duplicolor which is typically lacquer. The advantage of lacquer is its fast drying quality, BUT the finish usually looks dull after drying. The great thing about it is that the paint can be polished to an absolutely flawless glass-like finish. Same can be done with the high end automotive urethanes like DuPont or PPG with the advantage of durability and fantastic chemical resistance.
Old 04-14-2020, 11:50 AM
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Default Getting ready to paint




Yup, you summarized the issue perfectly!!
I do want the shiny, glossy surface of the original... A Extreme Flight 91" Extra 300.
The damage and area I need to repaint is mostly on the bottom front and under the nose. Mostly covered by the 4" spinner or under the nose.
I realize it's not going to be perfect, but I do want to match the white color and glossy finish.
I can't face 2 part paints or hand polishing.
I'll look for a spray can of lacquer....
not Rustoleum or the like??
Old 04-14-2020, 04:09 PM
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Originally Posted by dksnyder



Yup, you summarized the issue perfectly!!
I do want the shiny, glossy surface of the original... A Extreme Flight 91" Extra 300.
The damage and area I need to repaint is mostly on the bottom front and under the nose. Mostly covered by the 4" spinner or under the nose.
I realize it's not going to be perfect, but I do want to match the white color and glossy finish.
I can't face 2 part paints or hand polishing.
I'll look for a spray can of lacquer....
not Rustoleum or the like??
Rustoleum is an ENAMEL paint. By one definition, it's a paint that turns from soup to solid by two means, evaporation of solvents and a chemical reaction from being exposed to oxygen, ultraviolet, etc. After enamel has dried and cured, its chemistry has changed. The solvents that kept the paint liquid in the can no loger dissolve the paint back to a liquid after it's dried. The problem with enamels such as Rustoleum is that it's not particularly durable, but the greater problem seems to be how long it takes to fully cure. It can easily take several months for the paint to achieve its full hardness and chemical resistance. If you can wait that long, the paint can actually be wet sanded and polished to a fine finish. If you are counting on a glossy finish or the "wet look" from just spraying alone, it gets a little iffy using spray cans like Rustoleum. It is indeed possible to get a nice (but not extraordinary) finish from spraying alone using spray can enamel, but the problem is that the finish or the gloss is not predictable and can vary substantially between colors even from the same brand of paint. There is also Rustoleum clear that can improve the gloss of the finish, but don't expect anything spectacular.

Lacquer has the simple advantage of drying quickly and responds rather nicely to wet sanding and polishing after a few days of drying time. Lacquer is a paint that goes from soup to solid solely by the evaporation of solvents and remains chemically unchanged. That means that if you add solvents to dry lacquer, it will turn back to liquid paint eventually. It's like sugar and water. You can paint a surface with sugar water and it will leave a clear coating when it dries. If you add water again, it goes back to being sugar water. Lacquer has its disadvantages also. The solvents used in lacquer are chemically "hotter" than those used in enamel and as such are more likely to affect the surface to which it's applied. It will dissolve styrofoam, for instance. It can also cause the materials used in the repair area to swell slightly resulting in some definition given to the repair. This is because some materials will absorb some solvent from the paint and act like a sponge. Fortunately, whatever definition appears in the surface of the paint can be easily wet sanded smooth after the paint has dried.

In an attempt to make this bit of info just a little more complete, I have to mention automotive 2k urethane finishes. These are ones you get only at auto paint suppliers rather than auto parts stores or hardware stores. 2k means it has 2 components like epoxy and works in a very similar manner. 2k urethane is typically used with spray tools like a gun or an air brush, BUT at certain places they are also available in special spray cans that are filled and pressurized at the supplier. That means that they can make up any paint they carry in an infinite variety of colors in a spray can. Two main advantages of 2k automotive urethanes are durability/chemical resistance and an overnight complete cure. Also, unlike lacquer, the solvents are not as harsh so the underlying surface remains unaffected. Another advantage is that the paint remains as glossy when it's dry as when it was wet, making it unnecessary to polish in most instances. There are a couple of disadvantages too. The first of course is the price, the other is that if you opt for spray cans, there is a special button on the bottom of the can that releases the hardener into the paint within the can and that means that you have approximately 24 hours to use the contents of the can before the stuff solidifies in the can. And yes, urethane is an enamel.

Hope that makes some sense for ya!
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