A couple of thoughts for you.
First, Norm and Fatmantis are completely right. Sprue painting is almost always a bad idea for all the reasons they mentioned. Additionally, you'll probably want to dry fit the model before you glue it together or paint it.
Generally speaking, you always want to build and paint from large to small. So what do I mean, "from large to small"? When assembling models I mean it's often best to assemble the head, torso, legs, and sometimes the base as one piece before painting; then the arms each as another piece including shoulder pads and weapons. And of course any other pieces such as a back pack afterwards.
If this were a space marine for instance, once you have each arm and the whole body you can dry fit the model (possibly including blue tack) and get a sense for how you want it to fit together.
I find it is often easier to spray prime a model in these general sections as well.
Then when it comes time to start painting, you have sections of model that are easy to hold and manipulate, without too much model obscuring detail or making painting more difficult.
So then, 'painting large to small' is a process for using a larger paint brush and covering most of the model inn three largest areas of color first, including any washes, dry brushing, or blending.
When you do the big areas first you can afford to be a bit looser and sloppier, and of course it's harder when using a big brush working in large areas to be completely tidy. Then you should work with a fine brush filling in progressively smaller details such as eyes, picking out studs/rivets, scrolls/text, and doing any final blending, highlighting, or finishing touches.
Finally the base.
I hope that helps.
That's my process. I've been at this for about 20 years, and it works well for me. Best of luck =)