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reinforced bondo for quick cowl mold?

Old 02-20-2015, 01:39 PM
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flybyjohn
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Default reinforced bondo for quick cowl mold?

I have an old hobbico extra 300s that has a broken up abs cowl. I want to make a new fiberglass one but spend little money on it. I was thinking of taking the cowl off, cleaning it up, place patches over the cutouts in it and fill will spray foam to give it rigidity. I then would fill the plugged cutouts with a filler of some sort and sand it smooth with the rest of the cowl. This would be my plug. Then go through the process of building a two part mold like you would out of fiberglass, but instead use bondo with drywall fiberglass reinforcing tape to give it strength. With this mold I could then make a fiberglass cowl. I only need it to work once or twice if I screw it up. I have searched around and have not found anything about making a mold from bondo. I would think that since bondo is just polyester resin mixed with talc, that it would be similar to using polyester resin and fiberglass for a mold. I have already got the bondo and fiberglass tape to make the mold so I was going to give it a try, but wanted to find out why this method would not be used much if not at all. Any comments?
Thanks.
Old 02-21-2015, 03:24 AM
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No reason it won't work. I've made similar molds for wheel pants using bond. The only difficulties I see is that it will take a bunch of small batches of bondo to cover any decent sized cowl due to the short working time of bond. Also, it can sometimes be difficult to make sure there are no air pockets under the bondo when you're gobbing it on, but those can be filled/patched after the cowl is removed. Don't forget to wax your ABS cowl well.
Old 02-21-2015, 10:11 PM
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Thanks for the tips. The cowl that I am using for the plug is going to be half bonds being that it was so cracked up. I had already fiberglassed most of the inside to hold it together last year and any where there wasn't fiberglass, it was cracking again. I've almost got a complete plug now and will prime it. Will primer be good enough before waxing or should I paint it with gloss paint also.
Old 02-22-2015, 03:41 AM
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No need to paint. Just primer so you can sand a nice smooth finish.
Old 02-23-2015, 07:58 AM
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As another alternative, if you have access to someone with a thermo former, you could have a copy vac pressed from your original cowl. Then you could use the ABS or Styrene copy as a female mold. With a vac formed copy, you could lay up a fiber glass cowl inside the female mold with very little finishing required. If the original cowl is too large, you might have to cut it into 2 halves for vac forming.
Rick
Old 02-26-2015, 01:39 AM
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As a plug and mold maker.. a coat or 2 of paint will make a hugh diffrence.. we always paint and will even wet sand with 2000 grit paper..
Then several coats of wax and spay on pva then start making the mold.
Old 02-27-2015, 09:40 AM
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Thanks forall the advice. I am looking to keep this as cheap and with the leastamount of labor as I can. It is for an older plane so if I have to put alittle bit of sanding into the final product before painting, that is noproblem. Just as a test, I put 6 coats of carnauba wax over the plug thatwas primed with sandable filler primer and then wet sanded with 400grit. I mixed up 3/4 oz body filler with 1/4 oz polyester resin (knownas rondo) and then put some graphite powder in and the hardener. Mixedgood and then painted it on a rear corner of the cowl for a test. I hadover 10 minutes of working time and the mixture went on thick like honey consistency. I also put a 1/8" layer on top of two spray paint caps that had someraised labeling. It was a last minute test with the extra mixture so I lubedone with corrosion x and the other with some synthetic grease, wiped clean andthen put the mixture on. Once cured the mixture just about fell off thepaint caps and left a perfect imprint with no voids. I don’t think the caps needed any mold releaseas the resin just doesn’t stick to that type of plastic. The bit on thecorner of the cowl came off a little harder but did separate well and did notpeel the paint. The mixture when curedis fairly hard and smooth. My plan is todo a two half mold and to use this mixture for the first layer (kind of gelcoat) and then use bondo body filler foran extra thick layer over the top of this mixture layer using a bit of drywallfiber tape in the bondo to give it a little more strength. I did not feel much heat at all as themixture set up in the plastic cup I used for mixing which is good as I don’thave to worry about extra heat on the plug. If needed I can probably sand the mold with 600 to 1000 grit paper tosmooth it out a little more before laying in the wax. I have heard that some have used heavystrength hair spray as a pva with excellent results for small projects. I don't have a local source for pva and don't want to order some for this small project. I am planning to do one more test tonight onthe corner of the cowl using carnauba, hairspray, the RONDO mixture, and bondoover the RONDO with a piece of reinforcement in the bondo as a final testbefore making the actual mold for the cowl. This will give me a good idea of how much time I will have between stepswhen I lay out the actual cowl.
One moreside note, I have heard of using a vacuum chamber to get rid of smallairbubbles in resin before using in molds. I was wondering if I could use a canister and my vacuum sealer as avacuum chamber. Place cup with rondo incanister, hook up vacuum line, pull vacuum for a minute or two, then usemixture. Do you think this would work?

Thanks again for all the help and I will post some pictures of my try at the procedure whether good or bad.

Old 03-02-2015, 06:56 AM
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flybyjohn
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Hairspray as pva was a no go. In the end, the best thing I had on hand was Vaseline. I had to put it on, heat it up and brush it out. It did release the plug but I had brush marks in the mold. I sanded with 180 to 1500 grit progressively and then waxed the molds. They seem very solid and smooth now. Now I will have to do some tests to see what will work best to keep the epoxy and fiberglass from sticking to the mold. At this point I should have just purchased all the right stuff for the job as I probably have just as much money involved now that I would have with the proper ingredients. I do however have bondo to fix my son's car, a tub of auto wax to wax the cars in the fleet, hair spray for my daughters ballet bun, and a cowl mold that I think will work for my purposes.


I have seen a video of a demonstration where they laid up the two halves of the mold and then connected them together after both were cured. Do you know why they would do this instead of putting the two halves of the mold together and then laying up the cowl all at once? Also, does anyone vacuum bag cowls? It seams that you would get a better end product.

Last edited by flybyjohn; 03-02-2015 at 07:01 AM.
Old 03-02-2015, 07:09 AM
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Your spending way to much time on this..

This might help http://www.cstsales.com/tutorials/fiberglass_mold.pdf and http://www.cstsales.com/tutorials/fiberglass_parts.pdf

PVA is just an extra safety net, if you use a good mold release wax it will work also..

How about some photos of the old cowl (plug) so I can see what your doing.
Old 03-02-2015, 01:40 PM
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RBACONS
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PAM cooking spray works well in a pinch for a mold release. Spray on lightly and wipe off excess. However, a couple of coats of a good paste wax should have been sufficient.

Whether or not you join the mold halves and then do your lay up or lay up each half and then join them primarily depends on what you're molding. Wheel pants would be very difficult to lay up with the mold halves joined because you would be working through a very small opening. With a cowl, joining the mold halves first should not be a problem as long as you can align them properly. Usually alignment keys are created in the mold halves when the mold is being made so that the halves go back together perfectly. It does yield a better finished product (no seam to clean up and perfect alignment).

Vacuum bagging works better on flat or convex surfaces like a wing. It would difficult if not impossible to get the vacuum bag to conform smoothly the compound, concave curves of the inner surface of the cowl layup.
Old 03-02-2015, 06:04 PM
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flybyjohn contact me direct thru PM...
Old 03-03-2015, 07:11 AM
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Thanks PatternPilot for the links. It looks like just what I did except I did my cowl mold in two sides instead of top and bottom. I drilled about 6-7 holes through the mold before taking out the plug for alignment holes. I figure I can use 3 of them for alignment pins and the other 4 for screws to hold the halves together. When I have the mold clamped with 4 clamps, it will hold water for more than 20 minutes. I just get a drip out of it, so the sides fit together pretty well. I think I have about 6-7 coats of wax on it now (have been using Johnsons paste wax) and it feels smoother than glass. I will put some pictures of the mold halves on here tonight.

I had an idea that I think would work for vacuum molding and thought it might pull a little more epoxy out of the glass during curing. Just was wondering if anybody else has done it before. I will try and explain my idea in words although a picture of it tonight might work out better. Here it goes. take a large trash bag that will fit completely around the mold and open it up. reach inside the bag and grab a handful of the corner of the inside of the bag. With the handful of bag in your fist, put your fist inside the mold and push to the bottom. Then take the top part of the bag and pull it around the outside of the mold. Seal the end of the bag with your vacuum line sealed with it. Pull vacuum and there should be enough bag material you bunched up inside the mold to spread out and completely push against the inner walls of the mold. Now if you placed a few absorbent pads inside of some peel paper pouches inside the cowl against the walls, they would be able to absorb the extra epoxy being pushed from the glass.
Well does that give you a mental image of the process? I know I am probably over thinking things here but that's what I always do.
Old 03-03-2015, 02:45 PM
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flybyjohn
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Here are the pictures of the plug and the mold. I just buffed off the 7th layer of wax. One is slightly lighter than the other because I had less graphite in the mix. The dark spots up in the front of the molds are from a small repair I made when it cracked on taking the plug out. I am going to try and do a test with epoxy over the wax on another piece of bondo slab I used earlier for a test. Let me know if you see anything in the pictures that might be of concern. Thanks.
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Old 03-03-2015, 05:16 PM
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photos are worth a 1000 words... Mold looks good, Next time don't use bondo.. we can talk about that later... Do you have a an airbrush ?

Now there are two ways you can do the lay up,

Version 1 -

Mix a gel coat up , epoxy with cabosil to the thickness of thick of tomatoe soup, and brush onto mold.. if there are any shape angles make a thicker past and put into those areas and let it cure to almost hard, slight tack.

Next mix new batch of epoxy (only) and brush into mold and lay first layer if glass, i use 3 oz for this , use your brush to push down and make epoxy draw up thru the glass then add 2nd layer, i use 6 oz for this, if there are dry spots then add a bit more epoxy on brush and wet out the glass....remember more epoxy or a heavy wet look equals weight... do this for each half and now let epoxy harden, not fully cured and take a knife and cut around mold flush. now join the molds half together and use a 1 inch wide piece of glass cloth to join the half's together and let cure over night.

now pull the screws out and gently pry the mold halfs apart.. it should release without a issue.

It is hard to tell someone how to do it,, I watch several master glass people and have been doing this for the past 10 years.
Old 03-03-2015, 08:50 PM
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This thread reminds me of the Science Olympiad student who asked this question on their forum. "What is aerodynamics all about?" One of the best responses is that you could earn several degrees in the subject, and still not know all there is to know. The same is true for mold making, and composite fabrication. All of the advice in this thread has been excellent, but it only begins to scratch the surface. Here are a couple of pit falls you can encounter. Epoxy, and polyester don't fully cure for several weeks. Even though the surface is dry, it isn't done curing. This is why you can join the two haves of your cowl, and it will become an homogeneous assembly. What happens is that the joint strip and resin will co-lace with the "cured" on the individual haves. Now, why did I go to all the trouble to make that long explanation. It's because you don't want to put resin in a "green" mold. Even with wax, you risk bonding the part to the mold. I've made thousands of parts over the last 30 years, and I never lay up a part in polyester mold for at least the first month without parting film (PVA), then I'll layup parts with wax only. For epoxy molds, I always use parting film. To me it's just not worth aggravation of having a part hang up in the mold. I'm not sure Scott suggestion about prying the mold apart is a good idea. He probably forgot that you made an un-reinforced mold, and it might break. My suggestion would be to use air to "pop" your part out of the mold.
On another subject that you touched on; there are special films that are elastic, that are use in the vacuum bagging process. On a cowl such as yours, there's little or no advantages in vacuum bagging it. Companies like Fiber Glast Products have a series of instructional videos, that not only show mold making, but also introduce new products. Those are my tips for today. Good luck, Greg
Old 03-03-2015, 08:59 PM
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Greg,

your right i forgot it was only a bondo mold.. I use either eopxy and fiberglass cloth cut into strips for molds that I'm not going to do alot of pulls or use tooling resin.

We use a plastic pry tool for popping the 2 and 3 part molds apart. The canopies we just pull the corner and then it pulls out.

anyhow i always like to learn by watching and doing.
Old 03-03-2015, 09:25 PM
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Scott, A lot of the time we do things differently if we only want one or two parts out of a mold, versus a production mold, that you might cycle 3 times a day. Sometimes when a new-be ask a question, we give an answer based on many years of experience, when the guy only wants to make one part. I know that if I'm going to the trouble to make a mold, I'll probably go over board, just in case I need a couple of dozen parts later on. I liked your suggestion of priming, or painting the plug (repaired cowl). This is no different then applying a gel-coat, or surface coat. Then add a half dozen layers of cloth, and resin, and you have a mold that will last forever. The pictures of flyboyjohn's mold look good so far. I just don't want him to ruin all his work at this point, by having his part hang in the mold. If he can't find PVA where he lives, and he probably could if he would search out local business that do fiberglass work, then he would be better of painting the mold with a good catalyzed, or epoxy paint. Depending on the paint, he should give it plenty of time to cure. John, keep us posted on your progress, and if you have more questions, I'm sure someone will try to answer them. Greg
Old 03-03-2015, 09:31 PM
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if he has a airbrush I will send him some pva.. just waiting to hear.. yep everyone has there own way that works for them..

scott
Old 03-03-2015, 09:42 PM
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Scott, I prefer spraying my PVA too, but I've brushed it on a lot of times as well. Just use a good quality soft artists brush. You will get some brush strokes, but they are easily sanded out. One person on RCU suggested a foam brush, but I didn't care for it myself. Greg
Old 03-04-2015, 06:15 AM
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flybyjohn
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Thanks Greg and Scott for all the helpful advise. I don't have an airbrush, been meaning to get one for years but never have. I appreciate the offer Scott, but I think I might have someone in my club that might have some pva. They have been making composite gliders and parts. I will try and get a hold of them and ask.

I have read about the delayed curing in these molds and how the 4-6 % of resin is still not cured and will crosslink and bond even after the product looks fully cured. I do have a theory however that might play in my favor. When I built the bondo mold, and put 2-3 layers on top of each other quickly, the mold got so hot that I could not touch it for more than 3 seconds. I can only assume that the heat developed in the curing at that temperature did a faster more complete job of curing than if would have cured at only 100 degrees. Now it is only a theory and I hope I might be right.

I did reinforce the bondo mold with several layers of drywall fiberglass tape. It is still not the strongest of things but the tape does give it a little more strength.

I did a test last night using a previous test piece of the body filler/resin mixture with a slab of body filler over it. It still smells of bondo unlike my mold that no longer smells like bondo. It never got hot during curing. I waxed it 4 times and buffed completely each time. I laid 2 layers of 1.4oz cloth and then 2 layers of 6 oz cloth and since I had two more small pieces of 1.4 oz , I put them on too. This morning I pulled it off the mold and it released cleanly. It didn't just pop off but more like peeled off with a light pull. This piece was only 1.5" by 3 inches of fiberglass. A large cowl would have a bit more surface area and may be a little harder to remove. The air pressure idea would definitely be a big help.

Scott, you said you had two versions of laying up the mold. I assume version 2 is to connect the two mold halves before laying up the fiberglass inside the mold. That's the way I planned to do it. If it is not a good idea, let me know.

Last edited by flybyjohn; 03-04-2015 at 06:18 AM.
Old 03-04-2015, 06:44 AM
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I was thinking about putting 1 or 2 small holes in the bottom (front) of the cowl mold. I will put a small wax plug in both to seal from filling with resin. I then can both, use air to separate the mold from the open end and then also use a little air in the front end of the cowl to help separate the that part. I have a good feeling about this.
Old 03-04-2015, 11:47 AM
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John, Badger makes a cheap airbrush, that is based on the "bug sprayer" principal. I've used one for over 40 years for quick and dirty applications, including PVA, and even had some award winning painted airplanes done with one. I think they are about $20.00. If your hobby shop doesn't have one, try a Hobby Lobby craft store, or you could probably order one on line. A cowl mold is usually not too challenging of a layup. I have done them both ways: one layup in a single mold, or two piece mold; or two layups, either joined in the mold, or outside the mold. Since you have read, and studied up on mold making, you may already know that each part you make pulls a little wax off the mold surface, so you need to re-wax your mold from time to time. I usually re-wax about every three pulls, on an "aged" polyester mold. Second tip for the day is when making a polyester mold, do a build up of cloth layers over a couple of days. This will help avoid excessive heat which can warp, or distort your molds. If I'm reinforcing my molds with 6 oz. cloth, I only add 4 layers a day. I'm out of time for now. Greg
Old 03-04-2015, 04:10 PM
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Yes version 2 is what you said , but there is an order of doing that ... again way hard to type out, you might find a youtube video if you search . If yo need some pva let me know.. I will send you some. I will be at a rc show this weekend so i wont be on much.

scott
Old 03-05-2015, 05:58 AM
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Thanks Gregg for the tips and thank you Scott for the pva offer. I did find a local that has some pva if I want to use it but he has been laying up glider parts (wings and fuses) for a while now and looked at my mold and said I did not need to use it. "Just use the wax and it will be fine" I pulled my first cowl out of the mold this morning and it turned out ok for the first one. I was only after one cowl but I think I can do better. My epoxy was old and the 3 oz cloth was so tightly weaved that I think it was hard to get all the tiny bubbles out from under it. I used two layers of 3 oz, and two layers of 6 oz. I think I might skip the gel coat next time and use two layers of .75 and then one layer of the 3 oz and then two layers of the 6 oz. . I cut all the pieces out at a 45 deg bias and I think it helped lay down better in the corners. I did have a bunch of little air bubbles between the gel coat and first layer of glass this time around. For the first layer of glass, I wetted the inside of the mold and then laid the glass in dry and then wetted it. It was not easy. I started wetting the glass first and then it laid down a lot easier.

Here are some pictures of the first pull.

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Old 03-05-2015, 10:27 AM
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John, For a first effort it looks good. You now need to learn how to use "mud" in the sharp corners. The "mud" is just a mixture of resin, and some fine filler, like cabosil. (That may be a bad spelling of the word). You are a good study, which is evident in that you cut the cloth on the bias. Now for today's tip. All the fiberglass cloth made for composite construction has a finish on it. Without the finish, the cloth doesn't want to "wet out". Some finishes are made for polyester, and some are made for epoxy. Common names for finishes are Volan, or Saline. The good news is that most cloth made today have finishes that will work with both kinds of resins. I can see from the pictures that the heavy cloth is a conventional weave. For a cowl like yours, that has a lot of contours, you might have better luck with a unidirectional, or crow's foot weave. Finally, You are doing over kill with that much cloth. Now if your airplane needs nose weight, keep doing what you are doing, but if I were making that cowl, I would use one layer of 4 oz. and one layer of 6 oz. It's common in the industry to talk about the total layup by the number of ounces of cloth. Your first cowl would be an 18 oz. layup. My suggestion would be to shoot for a 10 to 12 oz. layup. If you haven't already visited thayercraft.com. check it out. Steve is a master of many different discipline's, including model, and full scale aviation, and an all around good guy. He will also sell you cloth reasonably, in smaller quantities. Today's last tip may be another one you have read about. "Post curing" will make your finished part tolerate higher temperatures. An hour or so, at 120 to 150 deg. depending on your epoxy. Without the post cure your cowl might get soft in the hot sun. With the post cure it will tolerate more heat. This would be a good subject to read up on. I use a light box for most of my curing, but occasionally use the kitchen oven, but be careful. Also keep in mind that your mold might not like all that heat, so be careful there too. There are many kinds of resins. What are you using? Good luck, Greg

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