I have not tried spar varnish. I also have not tried the Plexi-acetone mixture. I have read about the plexi mixture and have heard that there has been some success. It's just how much plexi do you put into the acetone? I also saw where people ask how much plexi to use and the answer usually is "until the consistency is similar to the polyurethane". The next thing is, where do you get the plexi? I saw that alamocdc and swirlman were either trying or using the plexi mixture. Maybe they can chime in with recipes or process. I want to give it a try, it’s just that it seems so hit and miss with how much plexi to put into acetone.
The Minwax Wood hardener blank was brittle, and darkened, however it did not fall apart on me. Maybe some sort of mixture between polyurethane and wood hardener would work good too. The polyurethane turned very smooth, however was darkened slightly. I want to give the Nelsonite and Polycryl another shot. I just got more cross cut spalted maple in to start more tests. I have been using only that blank, so as to keep the tests consistent.
The one thing I have noticed is that no matter what you use, you still have to soak some CA in at the end to really stabilize the end product and prepare for the CA finish. As you do this the spalting still bleeds a little.