USING PAPER MACHE FOR STRENGTH

by Rndurrant | March 15, 2021 | (6) Posted in Tips

 I’m in my late 50’s so reminiscing back to my grade school days spans a considerable amount of time. However I remember, as probably some of you do, making paper mache Easter eggs for grade school art class. I loved making things as a child, and must admit, I still do.

  While building one of the great Flight Test Master Build F4U Corsair RC aircraft, I searched forums, and internet articles for ideas to incorporate into my plane, and to learn from other builders. One thing I saw often was balsa wood being used in strategic areas to reinforce and strengthen the fuselage to enable a removable wing, and for other reasons. 

 

  I live in a relatively rural area, and purchasing balsa is either an hour and a half drive to the nearest hobby store, or wait a few days for an online order to show up. 

  They say necessity is the mother of invention, and in this case that certainly rang true. I needed to strengthen some areas on the craft, as well as I wanted to eliminate the seams between the separate fuselage structures for aesthetics. 

  As it happened, I noticed some paper mache sculptured lettering at a local craft store. I picked one of them up and considering the size it was extremely light, and stiff, and strong for a paper structure. The wheels began turning……

  It wasn’t long after that inspiration that I was applying paper mache techniques to the F4U, and the results were stunning! 

The technique:

  The lettering in the craft store was very stiff and strong. I needed to duplicate that process. In grade school we made our paper mache adhesive from flour and water. While that worked well for an Easter egg, it took forever to dry, and doesn’t have the strength, or durability I was looking for, it wasn’t going to work for this project. The technique however was sound. Replacing the flour and water adhesive with watered down PVA (Poly Vinyl Acetate) glue was the answer. Many of us know this product as “Elmer’s white glue”. But I don’t recommend Elmer’s white glue in this application. It will work, but there is a much better product that produces a better bond with the substrate, and it’s water proof as a side benefit. 

  The process is simply ripping strips of paper to approximately ¾” to 1” wide by whatever length will work for your needs. The paper strips are then drawn through a small puddle of prepared adhesive, and the excess glue removed. The wetted paper is applied to the surface you need to strengthen, by layering in different directions. You can also mold lightweight shapes and objects using this technique.

 

 

Adhesive:  

I found Titebond III Ultimate® wood glue to be an excellent product for this job. You can buy it at the local hardware store for about 6.00$ for 8oz. which will make a lot of Easter eggs! The mixing ratio with water is subjective, a little water goes a very long way. A little bit of glue, and an even smaller amount of water, just enough to make the adhesive thinner than it comes out of the bottle, maybe 5:1 or so. So go easy on the water!

Paper: 

  What to use for the paper structure? Newspaper makes a strong, stiff structure, because it’s close to raw paper, meaning it doesn’t have any coatings or additives etc. like some papers do, so it soaks up the adhesive well. I use it in places where it won’t be seen, because its finished texture is a little bit rough as well as it is a little thicker than other papers, which creates more work for finishing. So in the interior areas where I needed reinforcement, I used newspaper. 4 layers worked for me , but every application will be different. The Titebond impregnated paper adheres well to the bare foam of foam board, as well as the foam board paper, and when dry, it’s tough and strong. It is not completely rigid however, it’s flexible to some degree but adds a nice stiffness to foam board.  When cured, it cuts well with a hobby blade, it’s sandable, and takes paint nicely. You could even reinforce your paper mache with popsicle sticks, BBQ skewers, carbon fiber strips or rods etc. I think of it kind of like paper fiberglassing……… 

  Lay each subsequent layer of paper mache in opposite directions when applying, to give the structure strength and more rigidity. 

  If you tear paper with its grain, it will tear in long even strips. Slide your strips of paper through the glue/water mixture and squeegee off the excess on the edge of the container. You don’t want it to be dripping all over, just very well wetted. Apply it with a 1” wide paint brush to smooth out any air bubbles. This is critical for strength, any air bubbles will weaken the structure. The wetted paper can be formed somewhat around structures. You may need to make small tears here and there to help it go around objects etc. You can also adjust the width and length of the paper to fit the structure you want to mold or strengthen.  I apply a light coat of the glue mix directly to the bare foam or base structure, and between each applied layer of paper.                    

 I haven’t used it yet, but I’ve read that the blue paper shop towels work very well for strength. They have a thick, bulky grain structure and soak up lots of adhesive, and form nicely. It may be just what you need for your particular application, but might add some unwanted weight, something to consider.  

  For exterior finish paper mache, I used a roll of Walmart thin masking paper. You get a ton of paper on a roll, it’s quite cheap, and it’s thinner and smoother than newspaper.  You could even use some types of left over Christmas wrapping paper I suppose. The thin masking paper lays down nicely on the exterior, stiffening it as well as hiding the joints of the fuselage sections, and when dry can be sanded to make a nice blend with the structure. One thing I really like about this exterior application is, when applied to areas that have wrinkled, it stiffens these weak areas and hides the wrinkles. I found that 2 layers of exterior paper worked very well. 

  Make sure to scuff the exterior foam board paper with steel wool or 300 grit sand paper, and scuffing the exposed foam may be helpful also. The paper mache will tack better to prepared surfaces.  I used ultra-light sandable spackle to help feather the edges of the paper in areas where I wanted a really nice finish.  

 

  In these photos, you can see that the joints in the wings and fuselage are seamless and smooth. Unseen is the interior fuselage in the area below the cockpit and forward on both sides, which has been reinforced with news paper, paper mache. The structure is much stiffer and stronger than with plain foam board. I’ve also reinforced the location where the battery is placed to give durability to this area where it has high use from changing batteries. 

  The blue paint is simply Rust-Oleum® in the shade of blue that matches photos of the Corsairs that were aboard my father’s Aircraft Carrier (USS Boxer and USS Rendova) during his Navy tours of service of the Korean War.     

  There is no packing tape covering over the paint. I applied 3 coats of Quick Shine Multi-Surface Floor Finish® over the raw paint to give it durability and the sheen you see. Quick Shine can be purchased at your local Grocery Store for 5.00 to 6.00 dollars. Packing tape will add more durability to the structure if you wanted to go that route.  

  There are many uses for this technique. It could be used to repair and reinforce crashed planes to restore not only strength back to the airframe, but restore looks as well. You can Stiffen your foam board spars with this method, as well as mold super lightweight bombs, and drop tanks and other types of ordnance for war birds, or any other type of structure for your aircraft. To make a mold of a roundish object, simply find an item, such as a plastic or glass bottle etc., or carve the shape of the object you want to mold out of Styrofoam. In the case where you’ll need to remove the item you are using for the shape, ensure the glue won’t stick to it, by whatever means necessary, then paper mache over that object until you get the strength and thickness you want. Let dry. 

The Styrofoam can be left in place creating a nice piece of scale ordinance with a smooth, tough, durable shell over its surface. In the case where you need to remove the item you’ve molded over, simply slice the paper mache around the center all the way around and remove the two halves. Then glue the two halves back together. You now have a lightweight, hollow, strong shape of whatever you needed.     

  There is quite a bit of good information on line for this process. Be advised, that the glue dries pretty fast, so plan your layup process before you begin. Keep a wet rag handy to wipe your fingers on as you proceed with the layup. This will help prevent glue build up on your fingers and prevent you from sticking to your applied layers of paper mache, and pulling layers off of your structure etc. Obviously this technique won’t totally remove the need for balsa, but perhaps this article may give you some ideas to add a little strength and reinforcement, and perhaps detail to your next project, and maybe make a few Easter eggs with your kids, or grand kids.

 

COMMENTS

rmb519 on March 19, 2021
A video of this article would be nice.
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Burnhard on April 7, 2021
The Corsair looks fantastic. On my current P51 build I have been experimenting with light weight filler and foam armour to hide the fuselage joints and dents but this looks like it adds much more stability.
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Rndurrant on April 12, 2021
Thank you.
As I mentioned in the article, I use the light weight filler to fill gaps, wrinkles, or any surface imperfections before I apply the paper mache. That removes any air pockets or voids between the substrate and the paper mache. Air pockets are bad, they really weaken the application.
the paper mache, smooths and strengthens the joints. It really works well.
One thing that might help also, is before you cover the joint with paper mache, try to remove imperfections in the joint area, high spots can be ironed flat, or shaved down a little, and low spots can be filled with light weight filler as you say. then paper mache over the top, when dry sand it out and feather the edges. Any little lines can be filled with filler and feathered. With this method the joints disappear and are much stronger. I guess it all depends on how much time you want to spend making your project look nice. For me building the plane is a big part of the enjoyment of the whole thing.
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USING PAPER MACHE FOR STRENGTH