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Best way for sanding fiberglass over wing joint ?

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Best way for sanding fiberglass over wing joint ?

Old 07-29-2002, 06:26 PM
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Default Best way for sanding fiberglass over wing joint ?

I applied fiberglass tape to the wing joint, but have some ugly bumps at the edge of the fiberglass strip (all the way along the strip) which need to be sanded off before applying the trim tape. What would be the best way to do this ? Using a Dremmel tool (if yes, which one works best without harming the balsa) or just Sandpaper (what grid) ? Oh yeah, I used 30 minute epoxy to apply fiberglass if that is relevant.
Thanks for all of your input
Old 07-29-2002, 08:58 PM
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Default Best way for sanding fiberglass over wing joint ?

If its a big bump, rough sand it down as much as you can. I use spackling to fill the weave and feather the edge. Cant tell its there when done.
Edwin
Old 07-29-2002, 10:19 PM
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Default where

hmmm...yeah you are right, never thought of using a filler there. where do you buy that speckling ? is it heavy...i mean does it add a lot of weight. Sanding that epoxy fiberglass sure is hard
Old 07-30-2002, 02:15 AM
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Default Best way for sanding fiberglass over wing joint ?

Ulf,
Are you talking about the sewn edge of the fiberglass that runs the length of the tape?
I usually cut off any edging that is on the cloth or tape with a razor blade. Once you cut the cloth try not to handle it too much so it won't unravel. I would not use straight 30 minute epoxy to laminate with, they make finishing epoxy for this purpose. You should also thin the epoxy with some denatured alcohol. The epoxy should be thick enough to fill the weave but thin enough to allow it to soak into the balsa. If you apply it correctly you should not have to sand it very much. Apply enough epoxy to stick it to the balsa and fill the weave. Anything more is just adding extra weight.
Old 07-30-2002, 02:57 AM
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Default too late

thanks for the advise, but it is too late already. You got exactly the point by describing that nasty edge. I will try to sand it the best I can and will try some filler if necessary. I applied some tonight on a corner to see how it comes out. If it works, I will just work the edge with the filler (green putty) a little . Shouldn't add to much weight (hopefully). Will take your advise for next time though. I know it is really not that important since one side is always going to be hidden in the plane and the other only shows on the bottom (at least it is the lesser worst), but don't we all like to come out our planes the most beautiful and perfect as possible ? I think we all do.
I don't know what happen to me by not using the denatured alcohol ? I normally use it always to thin down that epoxy for coating on firewalls or cowlings !
Old 07-30-2002, 10:02 AM
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Default Best way for sanding fiberglass over wing joint ?

I never use epoxy for this. It's much too hard to sand. I always use polyester fiberglass resin as it sands much more easily. Then use glazing and spot resin for filler. It's made to be used with fiberglass and resin and it too sands easily. These can be found anywhere that sells auto body repair materials. I've been using Bondo brand.

The glazing and spot putty is also excellent for filling pin holes in fiberglass- that's what it's made for. I can't imagine that spackle-type fillers would adhere well to epoxy's and resins...

When laying down the fiberglass, I tack it down with CA so it doesn't move around when the resin is applied. I've seen kit instructions say to use CA for the entire job. but I've never tried it. That could get quite expensive!
Old 07-30-2002, 11:15 AM
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Default Best way for sanding fiberglass over wing joint ?

The spackling is from any hardware store, home depot, lowes, places like that. You want the light weight stuff, there is a heavy spackling also. They are normally side by side, the difference will be obvious. Since its epoxy, a small aluminum sanding bar with a medium grit aluminum oxide sand paper on it should work pretty good. Stick some masking tape on the wood next to the glass edge, that will help prevent any gouges in the wood from the sanding bar. BTDT, this fix works. Next time use finish reson. I get it from the hobby shop, sands good.
Edwin
Old 07-30-2002, 12:18 PM
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Default Old tricks are the best tricks, eh?

To minimize sanding and filling on any fiberglass surface, it's good to use a squeegee of some sort, to remove as much resin as possible. (The "industry standard" is to use the same weight in resin as the cloth weight) For jobs of this size, I use ordinary playing cards... available in packs of 52 each, and disposable.
Old credit cards, calling cards, or anything similar can be used with good results. Once this is done, another tip is to place a layer of saran wrap, waxed paper, or even Monokote backing material on the glass, and gently smooth it down. When the glass is cured, peel off the plastic, and the glass will have a pretty nice surface with not much sanding required.

Ta da!
Old 07-30-2002, 01:59 PM
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Default Best way for sanding fiberglass over wing joint ?

If all else fails, you could try laying 3/4 or 1 Oz. fiberglass over the entire area overlapping the ragged edges. This should help smooth it over a bit. use very little finishing epoxy for fiberglassing. You need only enough to saturate the cloth. If you can see any shiny spot over the FG, you used too much epoxy. You should still see the weave of the FG cloth clearly after epoxy application.
Old 07-30-2002, 02:04 PM
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Default Best way for sanding fiberglass over wing joint ?

It won't help you this time--only wet/dry sandpaper used wet will. next time, put a layer of Saran Wrap (the kind used to wrap food in) over the freshly installed fiberglass. You can now smooth it down with your fingers and leave the Saran wrap on until the epoxy is cured. When you remove the wrap, it will be as slick and smooth as a baby's bottom, no sanding required and perfectly feathered edges.
Old 07-30-2002, 02:19 PM
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Default Best way for sanding fiberglass over wing joint ?

when I lay on the cloth, just before applying,I dust it lighty with Elmers spray adhesive..as the cloth is light, it only takes a fine spray to hold the cloth in place for the thinned epoxy, or finish resin, go easy on the spray, I have used this for years and is much easier than fighting frizzles and buckles...
Old 07-30-2002, 02:39 PM
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Default Best way for sanding fiberglass over wing joint ?

Guys,
I notice when reading this and other threads on fiberglassing that many people use fillers over the cloth to fill the weave. This really isn't necessary. A surfboard is a perfect example of how nice a finish you can achieve with nothing but resin. I learned to glass by watching my surfboards being built as a teen. Once you laminate the cloth, all you need to do is apply a sand coat. This is just a layer of resin with sanding agent in it, applied with a brush or squeegy over the laminated cloth so that it lays out nice and even. When it cures it can be sanded to a smooth finish which, for our application, is ready for painting. The surfboard guys apply another coat after that (the gloss coat), which they again sand smooth and then polish.

Another good tip is to use a plastic film over the resin as Mike and Rodney suggested. I have seen this technique used in vacuum bagging a wing and it came out looking like glass. However, the plastic used was a little heavier than saran wrap so it would not wrinkle.

Russ.
Old 07-30-2002, 03:34 PM
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Default Best way for sanding fiberglass over wing joint ?

Ulf:

It seems you have been given a lot of good advice for next time, but no answer to your question on what grit of sand paper to use to knock down the rough areas. Use 80 or 100 grit to knock down the high areas, than wet sand with 320 or 400 to remove any of the scratches left by the heavier grit sand paper.
Old 07-30-2002, 06:20 PM
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Default Best way for sanding fiberglass over wing joint ?

Actually, when I used to build sailplane wings using FG, kevlar, or CF directly over foam cores, the standard practice was to sandwich a sheet of stiff mylar between the epoxy and the vacuum bag. Not only does this leave you with an ultra smooth surface when the mylar is removed, it leave a trailing edge so sharp you could split a gnat's ***** with it.

Okay, okay... I've never actually test it on a live gnat, but the sharp TE does make very aerodynamically efficient airfoils.

BTW, by "sanding agent" I assume you mean microspheres or microballoons to lighten the resin mix and promote sandability. Resin itself is too heavy to lay on for smoothing purpose only.
Old 07-30-2002, 08:41 PM
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Default Best way for sanding fiberglass over wing joint ?

Originally posted by Volfy
BTW, by "sanding agent" I assume you mean microspheres or microballoons to lighten the resin mix and promote sandability. Resin itself is too heavy to lay on for smoothing purpose only.
Volfy,
The stuff I was talking about (for polyester resin) is a liquid parafin that you mix with the resin. It makes the resin dust up when you sand it rather than staying tacky and loading up the paper.

The resin is not too heavy. You just need to spread it out thin with a brush. I do it that way all the time and it works great. I don't think resin will be any heavier than the gobs lightweight body filler or glazing puddy that people keep talking about. Here is a picture of a wing that used this technique on recently. After this cured it took me about 20 minutes with a random orbital sander to get it ready for painting.

Russ.
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Old 07-30-2002, 09:55 PM
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Default Best way for sanding fiberglass over wing joint ?

That is a very handsome wing panel. I use epoxy almost exclusively. The polyester smell gags me, even outdoor working on automotive application. Epoxy is not exactly "fragrance-free" but I find it much more tolerable. Plus polyester resin melts white EPP foam, so it won't work with foam core wings.

I may just be too overly cautious about weight. I guess it's a side-effect from the years of building sailplanes.
Old 07-31-2002, 02:32 PM
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Default Best way for sanding fiberglass over wing joint ?

Yes, polyester will melt foam cores, so if you are bagging your wings you have to use epoxy. But for sheeted wings, poly is fine. You just need to make sure that there isn't any foam exposed. As for the smell... well.. yes it is quite "fragrant". Then again, it is 1/3 the price of epoxy too which is the main reason I like to use it. However, I am working on a new mold of an Me-109 that will have openings in the hatch and wing saddle area so that I will have access to the inside during layup. I want to start using the "layup and seam all at once" technique to build my fuses. Poly sets up too quickly for this, so I will have to start using epoxy. But I suppose we are getting a little off topic here.

Russ.
Old 07-31-2002, 02:46 PM
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Default it wasn't too bad

Ok, bought the sandpaper yesterday put on my old building T-Shirt and went down to the bench ! I sanded and sanded and sanded, then applied some filler had a couple beer and mounted the engine mount on the fuse while the filler was drying and sanded and sanded and sanded again. It came out pretty good and you can't see any bumps once I applied the trim strip over the wing joiner. Thanks for all of your tips and techniques. Some of them really helped (specially the one indicating to lay maskingtape over the balsa, otherwise I would have probably sanded the balsa completely off ), and others will be definetely be used for future projects.
Old 07-31-2002, 10:26 PM
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Default Best way for sanding fiberglass over wing joint ?

Another way to hide the weave if you are going to Monokote the wing is to go back to the Home Depot and pick up a tub of Paint and Patch spackle and a roll of 2 inch wide blue long mask tape. The total cost should be about 8 bucks and you will have enough for many planes. Fill in as many of the bad spots with the spackle, let it dry, sand as smoothe as possible, then apply the tape on the top of the fiberglass, overlapping the seams so as to cover all the fiberglass. Once you have it all taped up, go over it all with your heat gun and press it down with a rag. It is now permenant and you can monokote right over the top of it and all that will show trough will be the tape seams. The Paint and Patch is easy to sand and I use it for all my planes. It does not shrink and I can fill even the nastest of cracks and it always does a nice job of feathering it in. I love the stuff and it's cheap. Use 220 grit to start and finish up with a 400 grit. You will like the results. Good Luck, Dave.
Old 08-01-2002, 07:52 PM
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Default Best way for sanding fiberglass over wing joint ?

seafury_fb11


I was wondering how do the glass job using the vacum bag system.

Ron
Old 08-01-2002, 09:58 PM
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Default Best way for sanding fiberglass over wing joint ?

Originally posted by filtr
seafury_fb11

I was wondering how do the glass job using the vacum bag system.

Ron
Ron,
I haven't actually ever done it (I still build my wings the old fashioned way. ). I've seen it done but that's about the extent of it. But, my wife and I just bought a new refrigerator because our old one died. I yanked the vacuum pump out of the old one before we had it hauled away so I could use it to build a wing bagging setup - one of these days.

Maybe Volfy can give you some details.

Russ.
Old 08-02-2002, 02:15 PM
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Default Best way for sanding fiberglass over wing joint ?

The concept of vacuum bagging is very simple. Instead of piling on books, bricks or cinder blocks for weight, you use the atmospheric pressure to press down the laminate on the foam core.

1. Prepare your foam cores as usual (LE, spar, servo cavity, joiner tube, end ribs, etc.).
2. Brush on a thin layer of epoxy first (I use West System, but many others are also excellent).
3. Lay on the FG/kevlar/CF cloth (I try to lay the weave diagonally for better strength).
4. Allow the epoxy to wick into and saturate the cloth, adding epoxy judiciously to dry spots, if necessary.
5. Lay a sheet a stiff mylar on both top and bottom of wing panel, ans slip the whole assembly into the bag.
6. Seal the bag (I use polyethlene plastic bought at Home depot, instead of the expensive vacuum bags, and seal with a "hot melt" type plastic bag sealer).
7. Turn on the vacuum pump and wait till epoxy is set up.
8. Remove cured wing panel from bag and peel off mylar.
9. Clean up the edges and shape the LE.
10. Voila!

I've probably missed a lot of details, but you get the idea.

Many more modelers use some type of wing sheeting such as balsa, plywood, obechi, or even brown paper bag (yes, the grocery store variety - it's free and works surprisingly well). For that, you can skip step 3-5 and just lay on the sheeting after the glue. I've heard polyurethane glue works very good for wing sheeting. It makes sense to me, but I haven't had the chance to play with it yet.

Unlike when I lived in SoCal 10yrs ago, I'm not currently into sailplanes so baggin' isn't my thing right now, so there is probably a lot more new techniques since I messed with it last.

This is a very very rudimentary description of the Art of Vacuum Bagging. And yes, it is an art. A well done carbon fibre wing glistening in the sun is quite a sight to behold.
Old 08-02-2002, 03:02 PM
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Default Best way for sanding fiberglass over wing joint ?

Volfy


Does the mylar go right on the epoxy. Do you apply a parting material. When do you use the Peel Ply material.

Ron
Old 08-02-2002, 06:10 PM
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Default Best way for sanding fiberglass over wing joint ?

I've heard of a variety of products being used, anything from gelcote release agent specifically for this purpose to PAM, the cooking oil spray.

Personally, I've had the best luck with spray silicone lubricant. I spray it onto the mylar and wipe most of it off. You only need a very thin film on it. Too much and you could mar the final finish on the epoxy. I've also tried a coat of good ol' carbuba wax (for car finish) and it works fairly well also.

My feeling is that the most important thing is not so much the release agent, but the surface of the mylar itself. You want to get ones that are absolutely scratch free and keep them that way. You let that epoxy get some "teeth" to bite on, and you'll need a lot more release agent to break them apart.
Old 08-02-2002, 06:22 PM
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Default Best way for sanding fiberglass over wing joint ?

Volfy

I am still wondering were the peel ply comes in.


Ron

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