Voids in Carbon

[OzBungy]

I am repairing the tip of my front foil wing and have re-built a damaged section with chopped carbon and epoxy.

It all comes up quite well except there are a couple of tiny holes in one section. I have cleaned the area with alcohol and ground out the surface of the voids with a Dremel.

I'm starting to lose count but 3-4 goes of clean, grind, fill with epoxy , cure, and after sanding the voids are still there. They look as though I had done nothing. Just a faint sheen of the new epoxy on the surface. I've tried both with and without chopped carbon.

Anybody got any suggestions on anything else I can try? We're locked down here due to coronavirus so I've got a few weeks to be able to experiment with repairs.

Plan B is to simply ignore the holes. They're pretty small. I've ordered some microfibre filler so I'll add that to ensure the holes are filled and laminate some carbon over the top and call it fixed.

[rynhardt]

If these are pinhole size, it's very difficult to get epoxy in there as the resin tends to seal the hole before the air can get out. I've had mixed success with a syringe and needle.
Alternatively you can try to thin your resin with acetone and see if that will be thin (non-viscous) enough to get in there.

[Si_B]

Sometimes a bit of heat on the epoxy during curing can bring trapped air out.

[plummet]

[OzBungy]

Thanks for the comments. I've tried needles, and heating the foil before applying the resin.

I've tried applying the resin then heating after cure has started. West says heating before cure can cause gas bubbles.

I've tried using chopped carbon as a filler. I've got a couple of West fillers to try, and West says to use them during laminating to fill voids. I guess if I do that I won't see the voids.

I've got some epoxy paste that I have used very successfully over the years for fixing chips and scratches. I would prefer to use the "proper" stuff rather than just fill things. That's part of the purpose of doing this repair. To ramp up my carbon and epoxy skills a bit.

[TomW]

If they are sub 1mm, then using filler compounds are the way to go. Epoxy is not really a good filler without some room to fill and fiber/ microballon filler. You want it really nice, paint it with a good primer and wet sand it.

[OzBungy]

My latest effort at filling the voids has worked. I did all the things I did before. Roughing the surface and scratching the voids with a nail. Heating the carbon with a hairdryer.

The additional things I did were to add microfibers to the epoxy. I smoothed the epoxy but did not cover it with tape or cling wrap. When it was well cured I gently sanded it with coarse paper. Just enough to get any bumps out and prepare it for lamination.

I am going to experiment with spray adhesive on the carbon to stop the carbon shredding, and see if it helps in moulding the carbon to the shape of the tip of the foil.

A few minor things. I bought some precision scales and a tiny silicone mixing cup and spatulas. I can measure and mix tiny quantities of epoxy. The last batch was 1.5 grams. It makes the whole process very precise and mess free. All the silicone bits can be cleaned after cure and reused.

These are the scales. They claim to be accurate to +-.03gram and precise to .001. My tests weighing coffee beans show they are very consistent. They fit neatly under a small sheet of cling wrap to avoid spillage. I started weighing out 6 grams of epoxy with my old scales (5:1 ratio). These ones allow individual drops of epoxy to be weighed. 1.25g resin:0.25g hardener.

[dice]

If these are pinhole size, it's very difficult to get epoxy in there as the resin tends to seal the hole before the air can get out. I've had mixed success with a syringe and needle.

Would it be possible to put the needle of an empty syringe in the hole, then fill the hole with epoxy (while the syringe is still there).
And then suck the air out of the hole, into the syringe.
So that there will be a vaccuum, sucking the epoxy into the hole?

[rynhardt]

If these are pinhole size, it's very difficult to get epoxy in there as the resin tends to seal the hole before the air can get out. I've had mixed success with a syringe and needle.

Would it be possible to put the needle of an empty syringe in the hole, then fill the hole with epoxy (while the syringe is still there).
And then suck the air out of the hole, into the syringe.
So that there will be a vaccuum, sucking the epoxy into the hole?

Depends on how patient you are? I haven't tried this yet.

[tahoedirk]

Thin CA glue and kicker/accelerator will work