See as it's a cowl from a well used airplane, there may still be some oil in the balsa? Apply some heat from a monokote heat gun carefully and see what if any, boils out. Heat it then wipe off what boils out with a paper towel and some acetone. If you used a lot of CA in your reconstruction, that won't soak up any dope. The garnet paper probably is causing you some grief also, even 220 grit,. Ditch that for some better stuff, for what you were trying to do, you didn't need to cut fast, just knock off the top. But now that you got it all stripped, I would still trying heating the problem areas to make sure there is no oil soaking. Then try your favorite method.
I don't know about this KBS Diamond stuff, as I recall mixed results from people. I read the description that Phil G wrote in Tight Lines that Howard posted, and using it as a base for sealing the wood was surprising, and that dope adhered to it, but his results showed it worked OK. good luck with it and keep us posted.
Type at you later,
Dan McEntee