Hate to suggest bondo, but since I went to the Doug Arnold school of Bondo and learned it's capabilities on the Isobar. It might work. But I find microballoons and 30 min or west systems works very well also.

I never heard of microballoons and K36 or K38. I might give that a try.

Also, there is a two part catalyzed "spot putty" that you can get at the automotive store. This is a very good putty and will fill in scratches and such very well and is certainly paintable.

Good Luck,

Sean