Glass fibre woven cloth and epoxy resin

[Chris Walby]

Thanks to all who have posted and in my case the Focke is already sheeted with quite thick balsa so there is no option to thin things out. What compounded this is the increase in size from 70 to 80 inch wingspan and a play it safe approach in the beginning with wood choice. I fully expect it to be a flying manhole cover, but (on this one) it won't be due to excessive resin!

Moving the tread on a bit with a related question.

What do people do regarding edges especially TE where ailerons/flaps locate? Tried overlap with both top and underside cloth, then just top wrapped down to meet underside without a great success. Tried clothes pegs or thicker resin at edge which seemed to trim better.

Best approach to TE, LE and fiddly corners?

[Former Member]

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[perttime]

If you overlap pieces of glass cloth, people seem to get neater transitions when they don't have a neatly cut edge. It seems to be easier to blend a torn edge.

[Andy Stephenson]

... No model covered in glass cloth has fully cured resin maybe at best it is 85-90% cured...

Barrie, I was interested to see you say that epoxy never fully cures, is this because it is spread so thinly that it can't take advantage of the exotherm effect it has when it sets in bulk.

Someone also said that epoxies don't cure properly when exposed to oxygen which of course would also apply if it's spread so thinly when covering models. If this is the case then applying a coat of water based hard glaze varnish over the epoxy, as I do, should improve the cure as this would exclude the oxygen.

[Former Member]

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[Martin Harris - Moderator]

I believe full sized FRP glider structures are heated to around 35 degrees C to cure them after laying up and reduce internal stresses. It's important that the structure doesn't exceed this significantly in service which could lead to deformation and is why they are invariably produced with white gel coat to limit heating by sunlight.

[Former Member]

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[Martin Harris - Moderator]

I thought that too as I was told by an experienced glider repairer/agent that the structure needed to be heated above any anticipated in-service temperature but I checked and found an article by the production manager at Glaser-Dirks which quoted just 35 degrees!

[John Stainforth]

I have a book on composite aircraft construction that says that the glass transition temperature (Tg) of most aircraft epoxies - at which they soften - is about 40F above the original curing temperature. However, if the structure is slowly heated above the original curing temperature, additional cross-linking of the epoxy occurs, which increases the Tg. Apparently, Rutan says that a completed structure, such as a wing, can be post-cured in this way simply by painting it with black primer and standing it in the sun for a day! Some people use a black water-soluble paint that can be washed off easily after this "post-curing".

Edited By John Stainforth on 26/10/2020 00:01:03

[Former Member]

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[Bruce Collinson]

This is fascinating.

I don't think anyone's mentioned peel ply on this post; does it reduce oxygen exposure? What do we think about diluting epoxy as Jon advocates? I've done it for fuelproofing front ends and tank bays but it feels counter-intuitive for glassing.

Also, I used some PolyC which I think is another iteration of WBPU and distinctly recall a statement that it wasn't suitable for wing joining bandages or, by implication, stressed areas. Shame, it's much cleaner and dries fast. I have used its Deluxe equivalent to skin control surfaces of solid balsa where I wasn't aiming for significant stiffening, just a more durable surface.

BTC

[John Stainforth]

I wasn't going to comment here, realising that one's opinions on modelling techniques depend so much on one's own experiences, but here goes anyway. I will restrict myself to my experiences in applying cloth and resin (not to later priming and painting stages).

Preparation of the balsa surface is essential, including filling where necessary with a range of fillers, from light spackle, to car filler, to resin plus microballoons. Sand the whole airframe, with #320 paper or lighter using sanding blocks, and tubes for concave areas. Use a vacuum cleaner and tack rags to remove dust from the surface, to ensure the resin bonds to it well.

For the fibreglass itself, 48 g/m2 glass seems best on most surfaces; 25 g/m2 on the lighter. The latter has a looser weave that uses about as much resin, so doesn’t save much weight. Aim to apply about 50:50 glass:resin.
Use Epoxy resin, which most real experts regard as far superior to polyester resin, which is much more brittle. (An epoxy glass surface does add greatly to the strength of a plane.) Use a limited working volume in one go: I found 35 ml about right for several fiberglass panels. Don’t dilute the resin to make it more brushable, because this can soften the cured resin (though some say up to 10% methylated spirits or isopropanol is OK). Better to decant the mixed resin into a container with a large surface area and minimum depth to prevent it heating up and thickening too quickly.

Before applying the resin, brush the cut fiberglass cloth panels onto the wood with a soft dry brush. This builds up static, which makes them stick to the wood. (Using a hard brush can drag the weave of the cloth.) The shapes of the panels do not have to be at all precise: I found it best to leave ample overhangs over the wood (e.g. about 2 inches) to prevent brushes or rollers picking up loose edge threads when applying the resin. A large overlap also provides something to hold on to, when you need to pull the cloth.

First epoxy coat. Wear latex gloves. Roll the resin into the cloth with foam rollers in a Union Jack pattern, working out from the centre (a la Nijhuis) – a fairly aggressive, fast approach works well (although don’t brush so hard that you break the balsa skin underneath!) Apply only enough resin to wet the cloth and stick it to the balsa, and no more.

Use wedge-shaped foam brushes to stipple the wet cloth into corners and around the edges (e.g., leading and trailing edges of wings). Trim the cloth overlaps back to about ¼” while the resin is wet with good scissors. (Remember to clean the scissors well, afterwards!) Make orthogonal cuts right up to curved edges, to facilitate brushing of the overlaps around these edges – which is best done with foam brushes (or the fingers, in latex gloves).

After 24 hours or more, use medium sandpaper to remove the surplus edges of fiberglass. Then very lightly sand the glassed surfaces with #220 to #320-grit paper to make them just smooth enough to apply the second ‘flow coat’ of resin. Remove dust by wiping and with tack cloths.

Apply the second (flow) coat of resin with foam rollers or a 1” soft brush. Rather vigorous, fast strokes work best. Apply just enough resin to fill the weave of the cloth and no more.

For sanding the resin flow coat, I favour 120 grade used dry. Oakey’s Liberty Green “fine”120 aluminium oxide paper seems to work better than any other and is most economical, because it is sold in 10 metre rolls. This sanding stage is a lot of work, because all the gloss of the flow coat has to be rubbed back until the general appearance is a dull, whitish beige with a satiny texture. This entails removing quite a lot of the flow coat and just going into the top of the fiberglass. Doing this dry allows one to see the shiny spots that need sanding. This best done out of doors, preferably in sunlight, which glints on the shiny patches. Don't worry about the fibreglass edges at overlaps - these get tapered away by the sanding until they completely disappear!

Finally, at this stage, I use 220 grade wet, cleaning off the residuum with damp paper towels, in preparation for the priming coats and further sanding, and sanding… and then the color painting.

[Martin Harris - Moderator]

A tip I was given, which works well, is to spray the lightest misting of 3M Photo Mount before laying (dry) glass cloth over the surface to be covered - it provides a very light tack to stop the cloth moving, along the lines of your static method, John - although it will still allow any small wrinkles to smooth out.

The thing that often amuses me is the lengths we go to to get a perfect finish on scale models. Compare this with the lumps, bumps and raw edges on the full size!

[John Stainforth]

Martin, I've tried the photo mount method as well, which is good, but I find the static method even simpler and easier. I think that was a recommendation by Phil Clark of FighterAces. He has some very good advice, because he does this everyday for a living, some of which he has put on the internet. I mainly used his fibreglass cloth, L265 resin and KlassCote paints on my S6b.

BTW, I think this kind of exchange of ideas and tips is where this forum is most useful - I love the build blogs. It's all those little do's and don't's that make all the difference!

[Andy Stephenson]

I find any kind of adhesive or static to hold the cloth in place unnecessary. I place the cloth over dry wood and pour on a narrow stripe of epoxy. Using an old credit card I gently squeegee it out from the centre. Once a small amount of the epoxy is spread this holds the cloth in place well enough to get more aggressive with the spreading.

There is a school of thought that applying spray adhesive even in a thin layer weakens the bond between cloth and wood.

[TigerOC]

I build foam core wings and have in the past used the wbp method. A guy in the club who worked in composites advised looking at peel ply. The last set of wings I have completed I used a new technique.

Preparation; cut the cloth and peel ply before starting. Prepare an appropriate size home vacuum storage bag. Ensure the surface is dust free. Have disposable/vinyl/latex gloves to hand.

Lay the glass cloth and apply epoxy laminating resin from the centre and squeeze out with a soft squeegee working outwards. I use a piece of depron. When done, apply the peel ply and go over the surface with your hands to make sure the surface is smooth and eliminate any creases or air bubbles.

Now place in the vacuum bag and pull a vacuum. Leave it for several hours. Open the bag and remove. Pull the peel ply and you will have a stippled super strong surface because all excess resin has been drawn out of the surface into the peel ply. It also cures much faster than just coating the surface with resin.

This provides a stippled surface with a nice sheen. It can be sanded to remove the stipples. The experts say this method is as close to vacuum bagging that you can get with simple tools.