Interesting that you bring up filling gaps. While I was trying to dry fit the aft castle, the hull got away from me and did a bit of crash and burn. There are sprung gaps all over the upper and lower hull joint. I am not sure how I am going to fix it at the moment. I had an almost perfect join but not anymore......
Anyway, what I usually use for this type of gap filling is Mr Surfacer 500 applied with Tamiya fine pointed Q-Tips. I let that dry about 10 minutes or so and then take a regular Q-Tip soaked in 91% Isopropyl Alcahol. Tis can get rid of the paint in the area but if you do it right it won't affect the grain and painting is a lot easier that trying to scribe new grain. After you get to where the gap is filled and there is no excess let it dry overnight and then apply more if there was shrinkage.
In regards to the sail, since this is a How To article/CD, I am building the Thomas Out of the Box, except for the stuff they call rigging line. I will scan the coat of arms decal before using it so that I can go back after the article is done and rerig with wooden deadeyes, etc... Pretty much like the Hanse Kogge from last year. If I can find a way to make realistic looking sails then I use the coat of arms I scanned. However, if I have the sail set, then I will need to populate the ship, otherwise it will look like a ghost ship in my opinion.
Hope this helps. I wrote an awesome reply earlier but it disappeared into the ether........
Steve