I was going to mention Min-Wax Wood Hardener, but my post was already too long, so I dropped it. My experience with it has been positive, but be forewarned: it takes a LONG time to fully cure and harden the balsa. I'm talking weeks here. Seriously. I have used it on some large balsa nose cones, and at first I wasn't at all impressed. It was only after I had left the cones hanging on their pegs for a couple of months that I found out how effective it is, and that was by accident. While reaching for something else, I accidentally knocked a large elliptical nose cone off its peg; as it fell (from a height of about 4 feet) it bounced off a couple of hard objects and then hit the hard tile floor in my rocket room tip first. If it had been an untreated nose cone, it would have suffered severe trauma. That's what I was expecting to see when I picked it up, but I was astonished to find that it didn't have a mark on it. A few months later, I had a similar accident with another large nose cone that I had treated with the Min-Wax product. Same result, only in that case the nose cone had fallen straight to the floor.
But the product is getting to be hard to find. It's been over a year since I last saw any on the store shelves. Also, it's incredibly volatile and incredibly, ummm, aromatic. (I always apply it outdoors.) And then there's that very long cure time. I am absolutely not kidding when I say that you have to give the object at least two or three weeks, if not longer, to fully harden. Balsa draws the stuff in very easily, so it soaks in deep. That's a big reason why it is so effective, and also why it takes so long to cure. After a day or so, the surface will look and feel dry, but that's deceptive. The interior of the piece will still be soft. If you try to use it before the interior fully hardens, the cone will get dinged and gouged up just as easily, if not more easily, than a similar piece that was not treated. Hardening the interior is a slow process. You must be patient with it and let it do its job.
Another thing about Min-Wax Wood Hardener is that it does absolutely nothing for the surface finish of the cone. It doesn't fill the grain, although it may seal it (eventually). You will still need to use other products to prepare the surface. That's why I like using laminating epoxy; not only does it harden the outside of the nose cone, it also fills and seals the grain and it creates a very smooth finish when it is sanded. So Min-Wax Wood Hardener is an excellent product, and nothing that I have ever used or heard about does a better job of actually making a balsa nose cone very hard and resistant to damage, and not just on the surface, but also deep into the interior. I have reserved it for use on my largest balsa nose cones, which are the ones that I am most worried about damaging. The two biggest problems that I have with it are: it's a PITA to use, and I can't find it anymore.
Mark K.