I feel sure this subject has been covered in the past but would like to hear from those that have added Castor oil to dope.
   I like to use silkspan and dope on my aircraft and have heard or read in the past that Castor oil in small amounts is a good thing.  From those that know can you tell me how this will improve the dope.  Also how much should I add and what pitfalls should I keep an eye out for.  Thanks in advance.
                                                              -  Alex

In the olden days, Testors was not plasticized, but I believe the Aero Gloss dope was.  Some of the SIG dope appears so. Not sure about the Brodak dope, but that is all I use and I still, at times put in a few drops of plasticizer for the top coats.  The brand I use is Dave Browns Flex-All (Flex 5100). Dave Brown Proiducts, Inc., 560 Layhigh, HJamilton, Ohio 45013.  The Flex 510 is for Acrylic Laquers, Acrylic Primers and Butyrate Dopes.

Plasticizer is used in dope to reduce the amount the dope film shrinks as it dries.  Sig, Randolph, & Certified make both tautening & non-tautening type dope.  Sig's tautening dope is called "Super Coat", and the non-tautening type is called "Light Coat".  Randolph's dope is just called "tautening" & "non-tautening".  Certified's dopes are labelled the same as Randolph.  All of the colors for all brands are non-tautening type dope.

Tautening type dopes shrink about 12% to 15% linearly; non-tautening types shrink about 5% to 6% linearly.  All dopes continue shrinking over time as the plasticizer leaches out & sublimes.  This is why you can rejuvenate a dope finish if the shrinking and cracking has not gone too far.  Rejuvenators are a solvent that contains film softeners & plasticizers to add flexibility to the old dope finish, enabling it to accept a fresh coat of dope.

Plasticizer, while it can contain castor oil (there is more than one type of plasticizer), has more than the one component in it.  You are better off, when plasticizing dope, to use a plasticizer like Dave Brown's Flex-all as Ty stated above because it contains additional components.  Plasticizer, if not overdone, does not interfere with drying, but it will yield a lesser gloss than tautening type.  Plasticized dope can be rubbed out to a high gloss, so the loss of some gloss is not a problem.  Also, if too much is used, plasticizer will decrease intercoat adhesion.  I follow the directions on the label for Flex-all.

I always cover with silkspan or silk because I like those, and for the first coats of dope on the open bay areas I use about 3 coats of tautening type dope, also on the solid areas except for the fillet areas and other concave areas.  For those I use non-tautening type dope.  For all of the build-up and finish coats I use non-tautening type dopes, and add plasticizer as I get to the final coats of clear.  I use Randolph dope for most of the colors because they have a huge variety of colors, for the final clear coats I use Sig because it is more fuel resistant than any other dope I have found.

This is what I have found works for me both in my full-scale antique/classic restoration shop & for my models.