Mark. Actually the best filler is Aluminum powder. It provides a high degree of UV protection. You might find it locally or order some from Aircraft-Spruce. Talc and cornstarch fill ok but offer no protection from the sun. While our planes are not subject to the same level of exposure that a full scale airplane might suffer, the fill properties of Al. are worth the trouble. I have used the powder on full size planes and it can be thinned so the dope is sprayable. It leaves a good finish after sanding and the weight gain is minimal. I think you will find dope and fabric to be a very rewarding undertaking. It is more work, but it sure looks nice when finished. Plus it is tough as all getout. Good luck and have fun with it.