We have had some success thining epoxy with denatured alcohol, acetone, laquer thinner, etc. I would guess that xylene would work as well. We have used syringes with wide bore needles to inject the thined epoxy and also poured thined into holes drilled into rotted areas. This has worked pretty well. Another trick is to heat the area to be treated and the wood will suck the solution in as it cools, the opposite of out gassing. We have done this in small areas around frame/chinelog joints that seem to be the favorite starting points for rot. Once the rotted area was saturated we used straight epoxy and then thickened to finish off the repair. We have been doing this on restorations over the last 7-8 years and so far it has worked well.
I don't have a lot of personal experience; however, I was at the System Three factory a few years ago. They had a demonstration of their rot fixing system that includes a product called ROT FIX. Among other things they impregnated a soda cracker with it. The cracker became the material that helped to bind the epoxy. You couldn't break it with a hammer.
Their system seems to be made to repair decks, houses, and other structures. I'd suggest you give them a call.
West Systems did research on thinning epoxy many years ago. They found that the thinner causes tiny holes to form in the dried resin, which let water in. It was not recommended.
A lot of wooden boat builders say the only cure for rot is to dig it out and patch the area with wood. Epoxy is not structural on its own.
As mentioned heat is a good solution.
For extensive rot, drill lots of small holes so the epoxy has more entry paths into the material. Heat the wood and use a thin resin and the slowest hardener you can get. I have not used West or System 3. The Raka 127 resin is a low viscosity resin.
After the wood is hot mix the epoxy then keep applying it until it won't soak up any more. If possible get the wood into a cooler place after the resin has been applied. Then add fillers to the epoxy and close up all the holes.
This can save some badly rotted pieces where replacement would be a big job.
I would only use a small amount of alcohol for thinning and then do a test cure.
Might take a look at SMITHS EPOXIES saves many a coner post that way. on an old sail boat.
SD MARINE EXCHANGE HAS IT IN STOCK
I've used CPES on several projects over the years, but it's always been on new wood.
Seven years ago, I saturated my unfinished mahogany front door on my house with CPES, and top coated it with a UV-inhibiting polyurethane. The door is totally exposed to the elements, no overhang.
It wasn't until this past winter that I began to notice any signs, whatsoever, of water getting to the wood, and that was where I had forgotten to fill the nail holes near the ground in the splash zone.
I've had similar experience with it, elsewhere, and am a fan enough to have just saturated the new bottom on my old driftboat with it. I also have a supply on hand for an old mahogany runabout that I'm about to get started on.
It requires no thinning. As a matter of fact, it goes on like water, no viscosity at all. You just keep putting it on until the wood won't take it up anymore.
As for its strengthening characteristics on old plywood, sorry, I can't answer that one; however, if you've visited the website, you've seen that the guy is exhausting with his product support verbiage.
You might also visit the Wooden Boat magazine forums. There's a good bit of discussion of CPES and its alternatives, there, too.
Gluvit makes a big mess, the few times I encountered it in repair jobs.
If you repair a structural area with epoxy...you have to put some strength into the epoxy. Some type of load carrying fiber or fabric. If you are just filling rot that doesn't carry any load, epoxy with any appropriate filler after going for best penetration on a first coat, that is what I do..I use acetone to thin it sometimes if I am going to put full strenght over the first coat.