Can paint thinner spoil?

Thanks Q, your 40 year old bottles make me feels better!  Yeah guys, thanks for all the replies, I suspected paint and/or thinner age were the culprits.  Bought some new mineral spirits and lacquer thinner.  Will give Testors gloss black a try this weekend with a Badger 100lg.

E

I'll cast my vote with those who suspect bad paint (although if your thinner is as old as you claim your paint is, the odds are that either (or both) are bad).  I have several bottles of MM, Testors and Humbrol paints that are almost 40 years old and often come across a bottle that is longer any good - Some of them clabber up and no matter how much thinner you use, you can't get the lumps out. Others will thin and apply normally, but when they dry they actually change to a different color.  About all you can do is to throw them away and get a new bottle of paint. 

We'll have to wait for Ross to chime in with the actual chemical engineering, but to my way of thinking, if the paint reduced normally, and sprayed normally, and appeared to be deposited normally, I would not think that the problem is the thinner.  If by fragile, you mean that the paint is not sticking well to the surface, I would think that the problem would rest more likely with either the paint or the surface prepartion.

By way of anecdote, I have a bottle of thinner that mostly dates from the 1990's.  I can from a 1 quart can of hardware store mineral spirits.  I use it mostly for cleaning, recycling it by letting the paint sediment settle into the bottom of the jar and then decanting the clear portion into a clean jar.  Although I almost always reduce may paint for airbrushing with new thinner, occaisionally I have used my old recycled stuff with no ill effects.  I would think that if thinner was going to lose its properties for reducing paint for airbrushing, I would have seen the effect with my old recycled stuff.

Andy