I learned a new-to-me technique over the weekend. I say new-to-me because others have been doing this for ages, and they call it "using a wash."
Sometimes when I'm painting Marine armor, my brush will slip, and I'll accidentally fill in a joint or crease. When this happens, I typically rinse my brush and attempt to pull the misplaced paint out of the joint. If I'm too slow, not all of it will come out and I'm left with a "fuzzy" joint.
Here's an example from my current plasma cannon Marine:
See that panel on his left thigh? I had trouble reaching it under the plasma cannon, and flubbed the lines repeatedly. It looks like crap, so I have to fix it. (Yes, there are mold lines on the hoses. No, I don't care. it's a metal cannon on a grunt trooper. I did my best.)
I get out my shade color and my bottle of flow improver.
I put a bit of paint on my palette, next to a pool of flow improver. Thin the paint to a desirable consistency.
Pick up a brushload of the thinned paint and dab it into the armor seams.
Not quite dark enough, so I use another brushload, but less thinned.
Much better. I hit this one more time at the same consistency, and allow it to dry. Then I carefully go back in with the topcoat color and clean up the area.
Done! The area goes from junk to acceptable in a few minutes. I used the technique on all of the mispainted or blurry joint areas, and the model looks much better now.
This isn't a mind-blowing revelation of technique, but it's new for me. I used to go back in and paint the seams by hand. Now, I just fill them using flow improved paint and retouch. The flow improver forces the pigments into the low areas, whereas paint thinned with water would create tide lines and not fill the joins.
Simple and effective!