With the initial painting done. (if you have deselected the layer just Ctrl+click on the painted layer that will auto create a marque around the silohuotte) Go into “Channels” and create a new layer under “Channels”. It will by default be named “Alpha 1”. Copy/Paste the painted layer you made into the Channel layer “Alpha 1”.
Now go back to the layers section, select the layer you painted on and go to: filters>rendering>lighting effects. At the bottom of this menu where it says “texture channel” select “Alpha 1”. Setup the lights to your liking. If done correctly you should have the beginnings of a very nicely lit object. I suggest using at least 2 spotlights one facing the object and the other back lighting it.
Select “ok” once you are happy with the results…and VOILA! Your layer should now have placed the lighting to that later.
The layer might appear a bit bumpy depending on how well you initially painted the map. If that’s the case I just use Smart Blur. It is under "filter>blur>smartblur.
Using the threshold, and a tiny bit of the smudge tool I get a nice smoothed appearance.
I hope this helps anyone out there…this is the least I could do seeing as all the many many tutorials I’ve read on cgtalk have helped my workflow and skillset improve by leaps and bounds.
Best of luck