I have some spalted punky wood blanks I'm turning into a stool seat. How do I stabilise it so it won't tear out during hand planing? It's to big for ca glue, if i use a wood hardener can I then plane it?
Minwax makes a hardener that will harden up punky wood to a point (your mileage may vary). I've used it and am a fan, particularly for water damage on a house where the wood is typically pine or fir.
The other option would be to buy Cactus Juice and stabilize with vacuum. They sell bags to handle odd/large sized wood, and of course, you would have to have some oven to set/cook the resin. Google Cactus Juice for more information.