If you are going to try castor oil, use medicinal castor oil. It is processed to a much more pure form, and is clear. Dave Brown products sells their product called "Flex-All" and Windy used it in his finishes. I really believe that is all Flex-All is, medicinal castor oil. it looks about the same , and I did a small taste test and it tasted the same to me.. You do not need much, a cap full to a guart of thinned dope I think, and when I use Flex-All, I also use fish eye killer from the auto paint store and had no issues.
Type at you later,
Dan McEntee