Hi Brad ;
What scale is it ? The reason I ask is this .The tiny guys are harder to do this to . I usually take the sails and very carefully seperate them from the yards , Unless they are already separate.
Then I take the sails and sand them smooth . You wouldn't see the panel and clews at this scale anyway ! Make sure the sails are glass smooth !
Now , take a thinned version of Elmers or Aleens and coal the sail evenly and place a piece of silk-span or tissue on the sail and GENTLY stretch the paper to fit the sail shape . Now coat the whole sail with the same stuff .
Let dry at least three or four days . Carefully remove the paper from the sail thusly. Separate one corner with an X-acto style knife . Gently blow in between the surfaces .It will eventually release giving you a semi soft sail that will keep it's shape under weathering and paint . T.B. P.S. if it gets out of shape before you paint or weather . Put it back on the plastic sail and spritz it with a wee bit of water .