|10-15-2008, 09:47 AM||#1|
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I am: Cliff S
Join Date: Feb 2007
Location: Belleville Ont.
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Old fiberglass fuse
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I have a couple of fiberglass fuselages, they have to be at least 10 yrs old. When I squeeze them they creek. I ran into this a few years ago when rebuilding an ultra-light aircraft. I was informed that there is over 200 formulas for the resins in fiberglass and a lot of them do not stick well to each other, plus they produce wax that comes to the surface during the curing process. The procedure I used was to wash it with a good detergent and water, dry it, washed it with lacquer thinners, then brush it with a wire wheel and re-vacuum. After all was said and done and the repair complete and cured, a sharp blow to the outside of the fuse would separate the bond. this is not really the bond I want on a firewall! I remember when I painted my boat many years ago there was a liquid that that softened the fiberglass in order for the paint to stick. Maybe one of you guys that work with fiberglass can shed some light on this?
|10-15-2008, 11:24 AM||#2|
I am: Boolean21
Join Date: Sep 2002
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Re: Old fiberglass fuse
If you are making a structural repair or adding a structural piece, you have to remember that there is zero strength in resin alone. If you are fixing a crack or maybe a separated seam on a fuse, you will HAVE to use cloth with the resin.
Also, your wire wheel is not much use for roughening up the surface. Sandpaper is the only good way to do this. The heat generated by a wire wheel will do not much more than gloss the entire surface over again and give you zero bond.
When doing anything with fiberglass, be it a repair or gluing in a new piece, my prep is always the same.
1. Wash the fiberglass with very warm water and Tide laundry detergent (powder, not liquid, it cuts grease and wax very well)
2. Wipe the glass with acetone or an automotive paint degreaser like PPG DX320
3. Sand the area that needs to be glued with a minimum of 150 grit paper. I generally start with 220 as it breaks the resin "skin" easier, but then go to 100 grit for a really good tooth. I will typically sand until I can start to see the actual cloth under the resin showing through.
4. Blow the dust out with an air hose.
Then there are multiple resins you can use to make a good glue joint. Even epoxy will work over polyester fiberglass IF you have prepped it properly.
When I apply my repair this is my steps.
1. Cut a piece of appropriate weight cloth to cover the repair. Make sure it's oversize. I generally make it twice as big as it needs to be, it gives a bigger footprint to distribute the load.
2. If it is a very small repair, I'll hold the cloth in place and saturate it with thin CA. This is often enough. If needed I'll sand that once cured with some 220 and put a coat of resin over it. If it is a larger repair, or I am gluing in a former, I will first brush a coat of resin into the area that needs the cloth. Then press the cloth into that area. I use the brush to wet out the cloth. You shouldn't need much more resin than what you've brushed onto the fuse to start with, but if it looks a little dry then put a bit more resin on the cloth. Be sure to push it in good, especially in a corner, or over a crack, you don't want an air pocket formed in there.
3. If you are in an 90 degree corner (ex firewall to fuse side) the cloth will not want to fit into the corner tight. That being the case, I will mix some resin with micro balloons or milled fiberglass and first put a fillet into the corner before I lay the cloth over it so that I don't have any voids.
Usually when I glue in a firewall or formers though I don't use cloth. I will use a couple of different products. When gluing in a firewall, I put a small bevel on both sides of the firewall edge. About a 1/16" bevel on a 1/4" firewall. Then I tack glue it into the fuse.
I will then create a nice smooth fillet with either resin mixed with milled fiberglass (not micro balloon's, they don't offer any real strength) or using a product such as BVM's aeropoxy, or an autobody glue that I use regularly called Plia grip. Once a good sized fillet is in there (generally I go about 3/16") with that bevel on the firewall it will never come out.
I wouldn't want to use anything that actually softened the existing resin, if it softens it, it could distort, it may never properly harden again etc. There are adhesion promoters for paint that will etch the surface, but I wouldn't use those for structural area's, nothing more than paint. When it comes to cured fiberglass, the mechanical bond that you get from proper sanding preparation is key. I have never had a former, firewall, etc break loose from a fiberglass surface, even in a crash, it will tear the glass all around it if the surface is prepped properly.
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