I am repairing the tip of a hydrofoil and will wrap a little carbon around it to reinforce the broken section. I've done it successfully in the past using the conventional method (wet epoxy, wrap carbon, peel ply, vacuum bag ...).

I've watched some videos on skinning stuff with carbon and they let the epoxy cure to the tacky stage before sticking the carbon to it. That sounds like a neat way to get the carbon to stick to an odd shape.

There's some similar videos showing repairs to carbon bike frames, letting the patch get tacky and using plastic sheet to handle the wet patch.

Does this process weaken the bond of the carbon?

Is there a risk of the carbon being not properly wet out when putting the subsequent coats on?

Any other tips or tricks from experienced users?

Thanks

Sounds like its for those who dont vac

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Mark _australia said..
Sounds like its for those who dont vac

True, but it can be hard to get the carbon to conform to the shape of the tip and wrap around properly and have it stay while you're applying the peel ply and absorbent layer and getting it in the bag and all that. Having it stick to where you put it could be helpful.

Sticking the patch to a sheet of plastic allows both the patch and the surface to be repaired to be properly coated with resin and go off at the same rate, and gives you the plastic as a handling surface.

Having tacky resin would remove the effect of the peel ply absorbing excess resin and passing it through to the absorbent layer, which is a big benefit of vacuum techniques.

I guess I could do a hybrid method by setting up for a conventional repair and do the steps of tack laminating in parallel with scraps of carbon and leftover resin.

In boat building we use the tacky epoxy method a lot, especially when laminating overhead. Most standard epoxies will have a reasonably long tacky time before getting to an unworkable state.

Wet the cloth out at the same time as the repair surface on a seperate board. Have a coffee and wait until the fibre is sticky as (a bit hard to get of the board) then peel away and bend it into place working it with your fingers. Very important both surface are very sticky, bonding will not be affected. If you want to add peel ply using his method, I usually mix a second brew once the fibre is in place an holding, lightly get some resin on the surface and brush out the peel ply on the job. End of the day, it's all about timing the brews.

Note. The down side of not using a vacuum is the laminate will end up thick due to the extra resin and no global pressure on the wet laminate. Only an issue with thin details where you want as much fibre as possible... like tip repairs...

I gave the tack method a go and wasn't all that happy with it. The basic method works ok, and it's good to know a different technique. It's not so good for doing a tiny lamination around a sharp end like the tip of a stabiliser.

The carbon slipped off while curing. That's operator error because I clamped the loose edge of the carbon and there was some movement at some point in the curing.

The how-to videos say you're supposed to use spray adhesive to secure loose ends, but my piece was way too small to do that. You're also supposed to mix fresh resin every 2 hours and apply additional coats to ensure a chemical bond, and glue down the remnants of the carbon. That doesn't work when there's wind in the afternoon and I want to get a session. It's also a problem keeping the work area in the garage warm for hours when it's 12 degrees. I'm using West 206 slow hardener and it takes 2-3 hours for the resin to get to the tacky stage.

I've gone back to my normal method for doing foil tips. I sewed up as sleeve of peel ply, the exact shape of the tip and quite snug. Wet out tip and the carbon with resin, wrapped the carbon over the tip and pulled on the sleeve. The sleeve squishes the carbon down to conform to the shape of the tip.

Add a bit of absorbent layer to absorb any excess resin (a bit of remnant quilt batting), pop it into a garment vacuum bag. The package is nice and clean and can sit in the corner of the living room where it's relatively warm.

In this case there'll be a couple of little folds in the carbon where the rectangular carbon folds in to fit the relatively pointy tip, but they'll disappear with a little bit of sanding and fairing.

I don't know why people don't use peel ply more. Everything i've heard from others, it seems to be regarded as an "only when vac bagging" thing. I find its perfect for when not vac bagging, because it allows you to push all the air out of the repair layers and get a solid repair. It also removes excess resin from the top of the repair.

I use it, even if i know im going to be sanding the repair area afterwards, because it creates a flatter surface to start with.