I agree with painting order: white first, then darker colors. White color is never white enough on darker background. Or, you might need at least four or five layers of paint. This might become too thick. But, on the other side, in reality, white was applied over original painting on true aircraft.
I use Tamiya adhesive tape for about 2 years. It's sold in various widths and is very good in filling gaps on surface of models. Leaves no glue trace on previously applied paint, no matter acrylics or enamels.