Best way for sanding fiberglass over wing joint ?  
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Best way for sanding fiberglass over wing joint ? - 7/30/2002 12:26:43 AM   
Texas 3D



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From: Houston, TX, USA
Status: offline 		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

< Message edited by Texas 3D -- Jul 29 2002 9:31PM >

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Ulf
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Best way for sanding fiberglass over wing joint ? - 7/30/2002 2:58:26 AM   
Edwin


 

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Status: offline 		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

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where - 7/30/2002 4:19:15 AM   
Texas 3D



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Status: offline 		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

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Best way for sanding fiberglass over wing joint ? - 7/30/2002 8:15:39 AM   
Giant Scale



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Status: offline 		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.

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too late - 7/30/2002 8:57:06 AM   
Texas 3D



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Status: offline 		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 !

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Best way for sanding fiberglass over wing joint ? - 7/30/2002 4:02:17 PM   
bokuda



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Status: offline 		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!

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Best way for sanding fiberglass over wing joint ? - 7/30/2002 5:15:17 PM   
Edwin


 

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Status: offline 		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

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Old tricks are the best tricks, eh? - 7/30/2002 6:18:33 PM   
Mike James



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Status: offline 		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!


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Best way for sanding fiberglass over wing joint ? - 7/30/2002 7:59:17 PM   
Volfy



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Status: offline 		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.

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Best way for sanding fiberglass over wing joint ? - 7/30/2002 8:04:19 PM   
Rodney



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Status: offline 		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.

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Best way for sanding fiberglass over wing joint ? - 7/30/2002 8:19:09 PM   
eugene


 

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Status: offline 		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...

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Best way for sanding fiberglass over wing joint ? - 7/30/2002 8:39:50 PM   
seafury_fb11



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Status: offline 		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.


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Best way for sanding fiberglass over wing joint ? - 7/30/2002 9:34:04 PM   
k_sonn



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Status: offline 		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.

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Best way for sanding fiberglass over wing joint ? - 7/31/2002 12:20:16 AM   
Volfy



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Status: offline 		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 a$$ 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.

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Best way for sanding fiberglass over wing joint ? - 7/31/2002 2:41:03 AM   
seafury_fb11



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Status: offline 		[QUOTE]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. [/QUOTE]

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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