Zero4Zulu

Zero4Zulu

Well Known Member
I'm working on rebuilding an older 6A...The cowl was never installed properly and the fit was terrible. I have it straightened out and have the new hinges installed and Mil Spec fasteners installed. Looks pretty good now.

Now I have a bunch of old rivet holes, screw holes and slots to fill. What is the best way to plug/fill these holes? Resin with flox or something else?

Also the lower cowl will need some major patches where the gear leg slot was cut. The plane had a nose gear failure before and was repaired poorly. The old patches came right off because there was poor adhesion. What is the best way to get the epoxy resin to adhere to the old fiberglass cowl?

Thanks for any help.

Steve
 
Flox works fine for filling the holes.

For the patches: Sand the area you want to work on. Wear disposable gloves, both to protect yourself and to protect your work from grease. Clean the sanded area from dust. I usually also run a rag with acetone over it, just to make sure all grease/wax/... is gone. You also want to wet the area with a bit epoxy before putting the first layer of glas on it. That's it.
 
For rivet holes, I would use flox to fill the holes, and a patch of BID cloth over that to secure it.

If this is a long run of rivet holes like a hinge line, you could use a BID tape to cover all the holes at once. Whatever way you do it, clean and scuff as needed.

For the repair of the nose gear slot, it depends on whether you have the old pieces to put back, or are creating a new part in total.

You mention that the previous patch came off----------this is one of the big problems with repairing old fiberglass.

There are three things that will affect the adhesion of a patch; cleanliness, surface prep--"Tooth". and resign compound used.

For a lower cowl repair, the cleanliness issue may just end up being a deal killer.

If this is a honeycomb core and prepreg layup, it is going to be extremely hard to get it clean enough to make a long lasting repair.

Do not be afraid to cut back material till you find an area that is totally free of oil inside the sandwich. You can use a flox mixture to close out the honeycomb, and just work off the exterior layup if needed.

Do you know if the cowl was make with epoxy or polyester??? You must use the same resin family to achieve the best repair.

For repairing a part when you have the pieces, fit things together as best as you can, I use a bit of medium CA to hold things in place. Then I put a single layer of BID on the outside of the part, to hold the parts in aliment, and give backing to the parts for the nest step....

Then, I go to the inside of the part, and grind a wide "V" shape at the joint area between the pieces. You want a scarf of at least 7 to 1 if using a backup layer, or 12 to 1 if not. I always use a backup ply or three. After getting the "V" areas ground in, mix up a bit of resin and add a small amount of flox----you want the consistency to be about that of honey. The flos is only there to help fill any voids, try to not let it build up where the surface is flat. Wet out the "V" areas, and lay in a single layer of cloth. Stipple it out well, and put another layer over it, slightly narrower as the "V" fills. Add straight resin to wet it out, stipple, and add another layer till you have filled the "V". Now let it all cure.

After that is all done, sand the repaired surface till it is all smooth and level with the surrounding base material, and lastly layup a ply or two over the repaired area, giving at least 2" overlap onto the undamaged area.

When you have the inside of the cowl structurally repaired, go back to the exterior, and sand the area you first covered with cloth, look for high spots or low spots. Sand out the high ones, and scuff up the low ones, and fill with dry micro. Give it all another sanding till all is smooth, and the edges of the overlay feather out well.

Done, ready for paint.

Good luck.
 
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BID patch

Your best bet for little holes is probably not to simply flox them in. Vibration can eventually pop them out. If it were me, I'd sand down the inside of the cowl at the hole. Apply a piece of duct tape to the outside of the cowl, over the hole, upside-down (silver side to the cowl). Stick another piece of duct tape over this one to hold it in place. Wet out 2 layers of BID at least 1" beyond any hole, so for a screw hole, maybe a 1.5" square of BID. Put it on wet, and then cover with peel ply (Dacron) and brush it smooth. When the epoxy is cured, pull the peel ply from the inside, and pull the duct tape from the outside and you should have a very nice patch that will be as strong as the original.

For larger holes, do the same, but I would apply the patch to the inside, and after it is cured, go back and put a 1 BID patch on the outside too. peel ply to make it smooth and easy to blend in with micro before you prime and paint.

The key to adhesion is to have it physically rough enough. 36 or 80 grit will be enough. Also, it must be clean. I try not to get solvents near composites, but in this case I would start with cleaning, perhaps Simple Green or something similar followed by water.
 
The nature of composites is such that the holes are unlikely to be weaknesses or crack initiation sites, so all you need to do is fill them in. So I would definitely not go nuts with the cloth when filling rivet and screw holes. The cloth adds thickness that you will have to blend in with filler, and next thing you know your airplane is a pound heavier. If I did use cloth on something like this, it would be a veil cloth of around 1oz/yd^2 that only adds around .002" thickness.

Anyhow, we use a good handful of clecos to hold composite glider parts together during bonding operations, and we just plug the holes with flox afterward. So far we have not had any problem with the flox plugs coming out.

Thanks, Bob K.
 
Thanks for the all the help!

I think on the bottom of the cowl, I'll attempt to rebuild the entire area including the old crooked gear leg slot. I'll grind out a V like Mike said. Hopefully I'll get below the oiled surface and create a larger area to meld to. There are some pretty large sections missing. I guess I'll need to make a mold surface to do this. First time for everything on this project! Rebuilds aren't what you think when you start!

Thanks again.

Steve
 
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