Testor ModelMaster Dark Sea Blue paint

No Ray, it is acrylic, which you can thin with water, alky, or Testors name thinner...none of it worked.

You didn't say wether the paint was acrylic or enamel. If it's enamel, try adding a few drops of laquer thinner to the bottle. I've never used acrylics before so I wouldn't know what to do about a problem with that kind of paint.
Ray

Could be contaminated, old (pigments and their associated solvents will degrade over time), or a processing problem at Testors with that particular lot. Save yourself the headaches, and plunk down a couple bucks for a new bottle...but don't purchase it at the same location as your current bottle. If it's a problem with particular lot, you'll be stuck again.
Gip Winecoff