Speaking of old build threads and the Humpty Dumpty, I use Bondo Spot Putty on plastic nose cones all the time. I have no problems with the solvents. If it's going to have to cover something thick, just do it in several thin applications with ample cure time between them and the shrinking/cracking issue is solved. An advantage to the Bondo is that you get an excellent surface that's ready to prime and paint. However (you knew that was coming), that was a lot of volume to fill, so the lighter HobbyLite filler was a good choice. I would have used whichever I happened to have on hand.

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I love sanding.