Hi - so a quick synopsis. I used a regular sea sponge from the craft store for the spongework. All the paints I used were acrylic because in my experience they work the best on most surfaces. The wood from the Uke was dark and grainy so I did a "primer" layer of plain white acrylic before i sketched and painted the rest. If you are painting an uke I think acrylic is the way to go. The iridescent paints I used are also acrylic (All of which can be found at a craft/paint store) Any more questions let me know !
goodluck
goodluck