The longer I paint the more I realize you need a good sketch before painting. When I first began painting I would just start slapping paint on a white surface and hope it turned out well. As you can imagine it wasted a lot of time and materials. Then I decided to start making some simple line drawings, that was a little more helpful but not when it came to the finished painting. i would find after i was done that I had forgotten some shadows or that my shadows were unrealistic. It help a lot to have a good sketch before painting. One you have a good sketch then apply a something to preserve the sketch like a thin layer of paint and then filling in the rest becomes much easier.
This is an example of a work done with a good sketch before hand.
This an example of one with out it.
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