Fiberglass + Foam: A Tutorial and Discussion

[willsonman]

Alrighty folks. Even before I was a guest on the new FT Community cast I had been getting this question. Since then I have been given the question nearly every day by a different person. Needless to say this is a topic that needs to be addressed in a public way.

So, I am going to go through this in a very detailed way. If you have questions SPEAK UP. There are no wrong answers and there are no dumb questions. At one point this topic was a black box for me as well. I will post some pictures as I do not have any specifically on-hand. So I am going to do this a bit off the cuff.

Materials:
Fiberglass cloth (lighter is better for finish work but heavier cloth is good for reinforcement but comes at a weight penalty)
Water Based Polyurethane (WBPU)
some sort of brush
Gloves (if you are that fastideous. You are using a water-based product so cleanup is fairly easy)
wax paper or some sort of drop cloth that you really do not care about

A word on fiberglass cloth:
Lets get the sourcing bit out of the way: Thayercraft
So different weights produce different finishes. The heavier the cloth the heavier the finish and the more filling you will have to do and more filling means more weight. It also adds strength so you have to work a happy balance. THe Volan cloth takes compound curves much better so for fine finish work use it where you can. Again, if its a finish layer, use the lightest weight (0.56oz/yd.^2). If were to strip off the paper and replace it with glass I would go with 0.73 or 1.43.

Ok, So start with your raw foam part that has been pre-cut to size. Dry peel off the paper from BOTH sides. If you dont, the foam will curl and it will not be flat. Repeat, PEAL ALL THE PAPER OFF.


I'm using Bondo brand cloth here to show the weave. This is STUPID heavy stuff reserved only for a combat-style airplane. Mostly this is used for auto body or repairing a kayak.

To the point... cut your cloth out to cover over the edges of your part.


Apply WBPU to the foam first... one side only. Place your cloth on top and smooth it out with your hand. Then apply more WBPU working it into the cloth. The picture shows dry cloth and wet. Bottom is dry. You don't have to go super crazy. You will likely use less WBPU than what you are seeing here. Heavier cloth requires more WBPU to wet out (increasing weight).


You can se I'm using an old crummy foam brush to apply. This is fully wet. Now to let it dry.


Drying can be done over a length of time or accelerated. My preference is using Hot summer days. The radiant heat and UV light from the sun will dry the WBPU out in just a couple of minutes. Since its colder here, I used my wife's hair dryer on high heat, low fan. Keep in mind you are using foam here. DO NOT use a heat gun. You run the risk of melting your foam. It took me about 5-8 minutes of constantly moving the hair dryer to get it dry.

Turn the piece over with the glass face down and trim the excess with a knife. The other way is even easier. For lighter cloths you can simply use 220 grit sand paper along the edge of the glass. It will sand right through it on the hard corner and you can just remove the excess. Since my cloth is basically ropes its not an option for me in this example.


This is what you are left with. Turn the piece over and repeat for the other side.


After this you can cut your grooves, control surface score cuts, bevel hinges, all like the paper was there. The glass will be more stiff than the paper, but that's the point and why you are here reading this.

If you are going this far I would highly suggest picking up your hot glue gun and throwing it in your drawer. Use white gorilla glue. It will adhere better to the glass and provide a longer-lasting airplane. IF you are gluing glass surface to glass surface... use a toothpick or a nail to poke tiny holes into the glass to give the glue a place to expand into and grip the pieces better.

I also prefer aluminum ducting tape to hold parts together while drying. It will stick on dusty surfaces quite well and also prevent the glue from expanding where you do not want it to expand to.

Up next will be surface finishing.

 

[Andre]

YES!

There goes my free time.

 

[earthsciteach]

I can testify to this method! As we discussed on the podcast, Josh sold me on this and I love it! It is easy, but not without work. The work is not tricky, but if you want a glass-like finish, be prepared to spend a lot of time sanding. I really like to sand, so I'm perfectly happy to do it!

I use even lighter cloth than Josh mentioned - 0.56 oz/yd from Thayercraft.com. I do have surfboard cloth that I used for a combat wing, but that thing is on the hairy edge of flyable. If you start doing this, next thing you know, you'll be wanting to build something with compound curves!

ALWAYS expand your horizons! Anything is difficult until you understand it. After you understand it, its simple!

 

[abieex]

I've been waiting for this. I'm going to order some material tonight! Could we turn this into a couple of episodes? Thanks Joshua

 

[willsonman]

Moving right along we have more supplies:
WBPU
baby powder (BP)
small disposable cup
something to mix with, I used a BBQ skewer
some kind of paint brush that will fit into your cup
I highly recommend some sort of respirator... even a cheap disposable one from the hardware store.

First you need to mix up what you are going to fill the weave with. This will fill those millions of holes so you can paint but also so that you have a finish that is as smooth as glass... well, if you put that much work into it. So pour about a tablespoon of WBPU into the cup. pour in about that same volume of BP and mix it up with your stirrer. Now add more BP. Now add more... keep going until you get to the consistency of a thick syrup. It will never get to molasses, it will be more like corn syrup. A bit thicker than maple syrup. If you get to the play-doh stage you went too far. Add a few drops of WBPU and thin the mix out. You want it thick to hopefully only have to apply one coat but you do not need it to be a paste.


Apply it to the glass with your brush and allow to dry. I'm using the hair dryer method here again.


So, when its dry it will look rough and bumpy and seem like you did that for nothing... That was just to FILL the weave. Now you have to smooth it.


Don your respirator. You really do not want to know what BP boogers look like. Here I am using 220 grit sandpaper (dry) knocking back the coat to where I can just barely feel the glass. It will be ever so slightly rough, kind of like a cat's tongue. In the picture below you can see on the right where I have sanded and on the left where I have not. Truth be told I did two coats of the mix here only because the weave is so big on this heavy glass.


All sanded down.


At this point you need to use multiple lighting sources and at different angles to inspect your part for smoothness. If there are still some low areas, repeat the above process.

If you are done, apply another thin layer of the mix and let it dry. This time, using 400 grit sandpaper (dry), you are sanding the ridges in the dry mix that are from your paint brush. The BP makes dry sanding easy so be gentle here. It does not take that much effort. Below is after my dry sand.


Now, you can keep repeating the process until you have the perfect surface. Keep in mind that this all adds weight. The more you do this process the better you will be at it and the lighter you can do the finish.

For the ultimate finish I highly recommend sealing the mix with an autobody primer. Then wet sand, wet sand, wet sand. I skipped the primer and did a wet sand on my demo piece here. I applied a bit of black spray paint directly to the finish.


And the result. Considering the glass I was working with this is not bad at all. Again, the surface will be what you are willing to put into it.


Top even out the color I would recommend a primer of some kind. Use white primer for anything using ligher colors like yellow, pink, or white. Use a darker (grey) primer if you are using darker colors or doing a warbird. It makes it easier to blend differing colors.

If you are completely bonkers, like me, you can add 1/64" chartpak tape where your panel lines will be. Apply thre light coats of primer over the tape. Once dry, wet sand back the primer over the tape until the tape is exposed. Then remove the tape entirely. You will be left with a tiny groove that looks like a panel line.

Not to be left out... I like rivets. Apply drops of wood glue as raised rivets. If they are flush rivets, get a small pencil-style soldering iron and remove the metal tip. insert a thin copper or brass tube that is the diameter of the rivet you are looking for. Plug in your iron and go to town. Just barely kiss the surface. It will not take much to imprint the circle into the primer and glass. I recommend doing a trial piece if its your first time. You do lose some depth of flush rivets after you paint. try different depths and see what you prefer. Then stick with that technique.

Latex, acrylic, rattle can, you name it, you can paint it. As long as the foam is sealed you can use anything you like. Any exposed parts or pinholes may risk the foam to being melted if you are using solvent-based propellants (rattle cans use acetone, that melts foam).

Keep in mid I did all of this as a demonstration in ONE evening, while watching kids with the wife out, and taking pictures. This is VERY EASY to do.

 

[earthsciteach]

I've been waiting for this. I'm going to order some material tonight! Could we turn this into a couple of episodes? Thanks Joshua

I would LOVE to do that. We barely scratched the surface. There is so much that can be done easily and inexpensively that leads to a really quality finished product. Joshua is uniquely suited for sharing this kind of thing.

 

[earthsciteach]

I'm printing out panel line drawings for the P-47 at this moment!

 

[wilmracer]

Great tutorial! I was intimidated by glassing for the longest time but it really is simple, especially with WBPU.

 

[Tench745]

I just did this to the fins for my 1/6 scale racer, apparently I sanded too much and when I primed the foam melted beneath the glass. -Sigh-
So, word to the wise, light coats on your primer and further from the surface is better.

 

[willsonman]

Too true Tench.

If you are trying to make the glass give a surface strength then the WBPU way is not for you. Really you should be using epoxy. Using epoxy requires gloves and an area that will get messy. The cheapest epoxy I have been able to find is Famowood Glaze Coat. Its perfect to try for the first time but it does have drawbacks. It take a very long time to cure at 72 hours at 70F or 21C. THe upshot is that mixing is easy with the 1:1 mix ratio. Be sure to do this by volume, not weight. The hardener is more dense than the resin and you will end up not using enough hardener. West Systems makes good epoxy for our use that cures faster and lots of modeling folks love it for how well it flows and saturates the cloth. I personally use Fibre Glast 2000 with a 60 minute cure hardener. All epoxy resins need a warm environment to cure out. So, if you have a relatively cold home I highly suggest making a hot box out of some foam sheeting and a simple incandescent light bulb. Its more or less like a very low heat oven.

The steps are basically the same as outlined above only substitute your mix of epoxy for the WBPU. You will use another coat or two of the epoxy to fill the weave. I also suggest that you invest in a smaller electric sander, unless you are like Patrick. If you are thinking that you can just use the WBPU and BP mix to fill the weave... just stop. Do not try it. The problem is that the epoxy cures to such a smooth finish there is next to nothing for the WBPU to stick to. I've tried pre-sanding the surface and it still does not work well because you are trying to get the mix to stick to the little pits where the sanding will not get to.

Another upshot of epoxy is that it does have some ability to penetrate and soak paper. I've tried using epoxy on DTFB and it does work but keep in mind that you are adding weight. Epoxy is heavier than the WBPU method but I am assuming here that the strength you are after is for a much larger airplane where the strength is needed.

The intent of the WBPU method is to provide a nice finished surface. This will allow nice paint schemes and fool others into thinking its a balsa model. It also provides "ding" resistance. Little bumps and blips from the hangar or transporting in your car.

 

[willsonman]

I want to expand on a precaution to those really wanting to try this.

Its building season in the northern hemisphere. Its much cooler than the summer months. When you take your model to the airfield in the summer do yourself a favor and keep it out of the sun. Try to minimize direct sunlight exposure to only when you are flying, especially if you are using darker paints. The head will cause the foam to slightly melt and release gasses that will form visible bubbles in your skin.

Use an easy-up, camo netting on some arrow shafts, or anything you have to to keep your airplane out of direct sunlight. I've had my models bake a bit in my garage with no issues but once in the sunlight, the story changes.

The way around this issue involves more work, much more. You would have to skin your foam with balsa and gorilla glue and then apply your glass. The reason is that the balsa provides a porous area for the gasses to escape to. It gives considerable strength and also eliminates the need for epoxy (unless you are a certain DC-3 being constructed by ScottyWarpNine).

Even when using straight up epoxy rather than WBPU keep your airplane out of direct sunlight.

 

[willsonman]

Ok, so now that the basics are out of the way I want to also make an honorable mention to a variation of this technique that I have demonstrated previously on this forum.

For some surfaces you may not want a mirror-like finish. You are just looking for ding protection and you could not be bothered with glass cloth. I use this technique to simulate fabric and hammered metal surfaces. These are typical for WWI era models.

Rather than use cloth and WBPU, use tissue paper and WBPU. You can pick up about 30 sheets of tissue for $1 at pretty much any dollar store where you might find gift wrapping. More expensive tissue paper has advantages that I will describe. The thing to note here is that you are dealing with a paper product applied with a water-based liquid. The water will cause the paper to wrinkle as you apply it. Some wrinkles may be too prevalent and you can sand them back and re-apply if needed. You can also work around compound curves using small pieces paper in a papier mâché style. Blending these pieces is again done with sanding after its dry.

After its applied you can paint with whatever and it does provide fairly good ding resistance. It is MUCH lighter than Peter's brown paper method with glue. Don't get me wrong, I like Peter's method, maybe for a very large boxy plane. For most folks on this forum, they are not producing anything as massive. Tissue will provide the protection you need while saving weight.

I will note that I used this method on the tail surfaces of my Sikorsky S-39 project (post #232). I did this to keep the tail light, lighter than using cloth, because I would have had to fill the weave as well.

 

[willsonman]

Joshua Finn brought up some good points on FB this morning and I would like to discuss vacuum bagging a little.

My experience is limited with this technique. There is a finesse to it that I've not worked on yet. Bagging requires mylar or some sort of thick plastic. The idea is that you wax the plastic with car wax, apply it to the wet epoxy and put the whole shmear in a bag that pulls a vacuum. What happens is that the plastic conforms to the wing and squeezes out the excess epoxy and pushes it into the weave for a perfectly even finish that is about as lightweight as you can get. Pull it out of the bag, remove the plastic, no sanding required for a glass-smooth finish. It can get messy in a hurry though. I have used space bags form a local discount store with success. For me, most of my wings are fairly compound at the tips so I would naturally get wrinkles I'd have to sand anyway. Some folks swear by it and having gauges that measure how many pounds of vacuum you are pulling. All that stuff really does not appeal to me so take it for what its worth. Anyone that has more experience with this please feel free to add.

 

[abieex]

Really good info here. I have been using masking paper from Lowes to cover wings etc. cut with my hotwire bow. I apply it with thinned carpenters glue and it works very well. Its sandable and provides a nice paintable surface but I will now try glass. Ordered my supplies last night.

 

[Craftydan]

Vac bagging does create some amazingly smooth finishes on flat parts like tail surfaces (and, yes, wings, if you're building them small enough to go in the bag) but I've had good success with a simpler method (much to Josh's chagrin -- he was doing the hard way and the results weren't that far off)

Wax-paper bagging.

Despite the name, the technique doesn't necessarily mean you bag in wax paper, it's just the material the people who've developed it used. You create the same layup in the "some sort of plastic" sheeting, then instead of bag and suck, sandwich the layup between 12 or more sheets of paper towels -- 6 on top, 6 on bottom -- then PRESS with a board and heavy weight (>20#/10kg or so). I'd recommend the blue shop towels, since the embossed patterns can show through. Let sit for the curing time, then pull it out.

The beauty of this method: No pump and no bags that require an unbroken seal for a majority of the cure time . . . and it can be done with cellophane wrapping paper if you're desperate.

It's not suitable for the thicker parts that vac bagging can still do, for instance a medium sized DLG wing, but even those are pushing it on bagging -- state-of-the-art starts using molds at that size. You do have to be careful in the layup to get the bubbles out, work the epoxy to where you want it and work the excess out, but you'll have similar tasks with layups using vacuum.


I've got some pics at home of the DLG tails I just finished -- I'll post them later today . . . 13.1g uncut for the pair, which is on the good side for a set . . . and smooth as glass

 

[abieex]

This thread just keeps getting better!

 

[willsonman]

That is why I made it! Great stuff Dan!

 

[Andre]

Wait the foam melts.

 

[Tench745]

Too true Tench.

If you are trying to make the glass give a surface strength then the WBPU way is not for you. Really you should be using epoxy. Using epoxy requires gloves and an area that will get messy. The cheapest epoxy I have been able to find is Famowood Glaze Coat. Its perfect to try for the first time but it does have drawbacks. It take a very long time to cure at 72 hours at 70F or 21C. THe upshot is that mixing is easy with the 1:1 mix ratio. Be sure to do this by volume, not weight. The hardener is more dense than the resin and you will end up not using enough hardener. West Systems makes good epoxy for our use that cures faster and lots of modeling folks love it for how well it flows and saturates the cloth. I personally use Fibre Glast 2000 with a 60 minute cure hardener. All epoxy resins need a warm environment to cure out. So, if you have a relatively cold home I highly suggest making a hot box out of some foam sheeting and a simple incandescent light bulb. Its more or less like a very low heat oven.

The steps are basically the same as outlined above only substitute your mix of epoxy for the WBPU. You will use another coat or two of the epoxy to fill the weave. I also suggest that you invest in a smaller electric sander, unless you are like Patrick. If you are thinking that you can just use the WBPU and BP mix to fill the weave... just stop. Do not try it. The problem is that the epoxy cures to such a smooth finish there is next to nothing for the WBPU to stick to. I've tried pre-sanding the surface and it still does not work well because you are trying to get the mix to stick to the little pits where the sanding will not get to.

When using epoxy, you want as little resin as possible for weight and strength. So apply enough to wet out the cloth and then using a playing card or piece of sheet balsa scrape as much epoxy as possible back off. Let this cure. Do one more coat like this but do not try to fill the weave with epoxy it will just be heavy. Two coats of epoxy should be enough to seal the foam, but be careful, you may have missed something.
From here you prime. Spray a light coat, sand off until you see the glass coming through. Spray another coat, preferably of a different color so you can see where you've sanded and where you haven't. Continue doing this until the weave and any imperfections are filled. Paint is lighter than epoxy. A final primer coat will even the coloring before spraying or brushing on your color coat.

I've used micro-light filler for the first time on my last build and I like it so much better than lightweight spackle. The texture is finer and it seems to adhere better. I get my epoxy from Duckworks Boatbuilding Supply because I also use it to -gasp- build boats.
If you only glass every once in a while don't buy from here. The smallest kit they sell is 3 quarts and you'll be mixing batches in the 1-2 oz range.
However, if you have a lot of glassing to do it could be worth it. Much cheaper than West-Systems with similar strengths. I recommend the fast cure. Shelf life is a few years. You can clean up uncured epoxy from tools and hands, etc with white vinegar; it stops the catalyzation process so don't get it on your part.

 

[willsonman]

Oh! a good point about shelf life! I've had my epoxy for years now. I got it out the other week and noticed that the resin had crystallized. Since it is in a metal can I just put it on the stove and heated it up while stirring. Back to clear, applied and it works just fine.

Yes, the squeegee does work and then follow-up with primer. If your aim is to save weight it does help.

Overall my recommendation to those tuning in is to glean what they are willing to try and then experiment and figure out what works for you.