J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Hull cracks at waterline

I agree that trying to dry this core would be a waste of time. While I have had OK results in some areas with heat and high vacuum the results are uneven. On my boat I have found it is just easier to peel the outer skin off and then evaluate whether or not the core can just be air dried or needs replacement. Often the replacement is very localized. But like today I just peeled the area under the galley sink drain and I think all of that core, from the through hull to the solid part of the center of the hull is going to end up being replaced.

To do your repair it is hard to imagine how get through it without some electricity. Renting a generator may be needed. There are lots of ways to cut the skin, but my preferred method is to use the 4" grinder with a thin metal cut off blade. I cut a pattern roughly 4" x 4" over the area I am working. Then you can use a flat pry bar and a hammer to peel the squares of skin off. If the sections are too large you really struggle to get the skin off. Once the skin is off in the area you are working you still have to get through the polyester resin layer. If the core is mush then maybe it will be as simple as scraping it away. My method is to again use the small grinder this time with some 36 grit paper is your friend. This is the hard part and you will need a respirator and long sleeves, hood or a Tyvek suit because the glass fibers are nasty. Once you are down to the end grain balsa a simple sharp chisel will let you slice and dice the old core up and shear it away from the inner skin. This is easy but tedious work. The inner skin is pretty thin and can lose its shape if you open up too large an area, but you also have to get the work done, so you make a choice and if you lose the shape you will just have to do more fairing at the end.

You have to clean the inner skin up too. If you are limited on tools then it is back to the grinder but use maybe 80 or 100 grit to get the remaining balsa and fillers off the inner skin. You have to be crazy careful with this inner skin or you will go right through it. I like to use a small pneumatic right angle grinder for this work with a 2" disc, but that will not likely be available to you, unless you also have a compressor. But you have to clean this inner skin up so the thickened epoxy can fully bond to it.

So once you have the old core gone in the area that you are working you next get to figure out how work against gravity. If you have a shop vac you can use it as the anti gravity tool. In this video below you can see how I use plastic and painters tape to seal the work area and then a simple sink drain as the vacuum port.

Rebuild

What I have found to be helpful is to use both Colloidal Silica and Micro fibers to thicken the epoxy in a rough ratio of 2 to 1. The reason I do this is that the micro fibers make for a much tackier mix so you can press your core into it and it will stay put for a while. In areas where I don't need the extra tack I just use the CS as the thickener. Once you have the core in place put your plastic over the work area tape it in place and tape your shop vac to the port (sink drain) and let it run. One thing you will have to decide is which hardener to use, fast or slow. For core work I normally use slow so the thickened epoxy has lots of time to move around and extrude to wherever it wants to go. But if it is not overly warm where you are working the fast would probably be fine and it would give your vac a break after a couple of hours running.

Then it is on to the replacing the skin.

I also suspect that the quote you got was very conservative. You really don't know what is going on until you get the skin off. And Chris is a great resource for this kind of stuff, he got me started and through a ton of this work.