Paint is made from three components. Pigment, binder, and a liquifier. The binder is any number of things such as latex, enamel, acrilyc, viynl. Certain binders only solidify in solvent and not water. Some solidify in both. From the sounds of your post ,you are looking to slow the dry rate ,evaporation, of the liquifier to prevent it from clogging your needle. Extenders are basically wetting agents and slow down evap. times. One of the best wetting agents that every one has easy access to is detergent. examples are windex, dish soap and a variety of other detergent based products. The best bet in my opinion for you would be to hobby acrilyc thinned with clear windex for your fine lines. There are no magic formulas for keeping paint from clotting up however. When you use hobby acrilycs you will need a good clear coat. Ideallly epoxy. Hope this helps.