Fiberglass work....

Started playing with the tip of the rudder today. Had to cut a groove in the forward face of the fiberglass to fit the front, had to sand down the back both for the notch and to have the right length of part.

After the drilling and dimpling and such.... i want to have a faired appearance and blend over the top of the pop rivets.

Reading all the fine posts and online information looks like I will scuff the part very well. Then mix a peanut butter consistency of flox/epoxy and seat the part in the top of the rudder putting this mix in the interface, then pop rivet to hold.

Then mix a little more and spread onto the joint on the outside using a tool to make it nice and even around the entire part.

After that sand it down, and then do a layer similar with micro balloons.

Then once I'm happy then two coats of plain epoxy to seal it all, with a 400grit sand of this to get the final effect.

Questions:
1. When in an application like this would cab-o-sil be used?
2. If pigment is used is it just in the final epoxy coats or in the previous coats as well? I suspect I don't really need pigment because I'll be painting later anyway.

Cab-o-sil will make the mixture thicker and won't flow with enough added. You should try a little to see, but having done a lot of micro-filler lately, you want it to smooth out a little as it is really hard to smooth out and not leave a really thick layer in places. I add a small amount of talc - pure talc from baby powder and it seems to allow some flow out and smoothing but did not seem to run as much as without. Maybe 1/3 - 1/4th as much as micro. Just added it at the end of the mix. It sanded nicely.

I have not found a happy application and sanding result with cab.

Flox mixtures don't sand well at all. I have used it for structural then micro over the top. It is primer thirsty too.

Color - may others have a different experience, but I have a lot of trouble making a perfect paintable surface with clear epoxy. Once sprayed with epoxy primer, it still needed significant work to look perfect. Color in the last two (last) coats might help that. Actually, two sounds heavy. Especially after making a nice smooth surface with the micro.

This is a good time to experiment with your material formulations and technique. I would probably stop at a good micro surface and prime with a good epoxy primer.

BillL said:

This is a good time to experiment with your material formulations and technique. I would probably stop at a good micro surface and prime with a good epoxy primer.

Click to expand...

Good advice; I've built 5 plastic airplanes and concur.

You are joining two dissimilar materials. To prevent the long term cracking bury a strip of glass cloth in your mix.
Every thing will look good for a few days or a few years, but eventually the heat and cold cycles that occur with the sun can cause a crack at the joint.
Be aggressive while scuffing the metal, sanding the edge down with a bit of 100 grit will also give you a better mechanical bond.
My preference is to fly unpainted for a year so. That these heat cycles have done most of their flexing and cracks and bulges can be fixed before paint. I see it every year at Oshkosh where beautiful glass work changes shape in the hot July sun. Tail/fuselage fairings are particulary susteptable to this aging process.

I agree with Tom that a strip of glass would be good. I have a small chip at each aluminum/glass leading edge. Mine have occurred from rain. I did use glass but sanded through most of it so it seems the rain has had an easy time of the micro.

Scuff the aluminum and glass joint surfaces, wet-rivet with epoxy/flox or a structural adhesive like Hysol in the bond line.

Later, scuff the exposed surface, fill the seam and pop rivets with West/micro.

Polyester/glass FRP (i.e Vans fiberglass tips) and aluminum actually have about the same coefficient of thermal expansion, roughly .000013 in/in per degree F.

520 hours now, no glass tape, no cracking.

3/32" pop rivets in 1/8 dimples make it easy to cover the heads.

Bond to Alu

If you want to bond any fiberglass to aluminum it is critical to etch the aluminum with something like DuPont 225S Cleaner just prior to coating the epoxy of course rinse and dry first.. Similar to when you do painting. I have found over the years and with a number of projects, rough sanded surface followed with etching provide for an almost indestructible permanent bond.