If you hand-paint an entire playfield, there absolutely will be texture. I've done it:
Pinside_forum_5430147_0 (resized).jpg
So be prepared to build up several layers of clear, such that it can be sanded smooth without digging into your painstaking work. Alternatively / additionally, you can buy a clear playfield protector for most titles. But you should still seal in the handiwork with a clear layer; protectors can and so shift and adhere slightly so you don't want that scuffing or removing the work.