Thicker lines and a harder brush should solve that problem, but using a bigger canvas can help out a lot as well. When it comes to size, as long as your computer won't explode, the bigger the better. I usually set my canvas size to about 3000x5900 to 9500x8700 max so I can squeeze in as much detail as possible, then I shrink it when I'm ready to upload. As for using the paint bucket tool, sometimes it is best if you trace the colors inside of the drawing, then use the paint bucket tool all WITHOUT using the magic wand tool because sometimes it will leave unwanted pixels and you will have to go through the trouble of cleaning it up x) I had that problem when I was using the magic wand tool myself, I got so frustrated with it that I just stop using it xD

Even so, you did a great job with the coloring ^^ and I really wish that SAI had an eclipse tool as well. I hate having to go to Photoshop just for one tiny detail ><

(didn't reply directly to your comment)

Trace the colors in the inner side of the lineart to enclose the bucket fill?

Thanks again for the tips.

Yep!! Even though you will spend more time painting, it will come out much cleaner that way ^^

And no problem man ^^