Thickened enamels.

[ewetwo]

My tail light red enamel paint thickened up. I’ve taken about half of the paint out of the jar in globs. Add mor thinner and it still is not thinning out. Do I just chuck it?

[peteski]

Enamel not only dries by solvent evaporation (like lacquers do), but it continues to cure (harden by chemical reaction) long after that. At that stage, you cannot just add some thinner to revive it.  If your enamel has clumps, that to me indicates that it has beyond drying out, and it has started to cure.  Not much you can do - chuck it.

That is the difference between lacquers and enamels.  When lacquer dries, it is dry and hard.  But enamels first dry, but remain soft, and it takes them long time to fully harden.

[ewetwo]

Thank you Peter. What about the ones that were never opened but you can see the pigment at the bottom and the solvent above? 

[peteski]

In a simple explanation paints consist of the solvent (liquid which evaporates as the paint dries), pigment (the actual colorant), and the binder (the liquid resin dissolved in the solvent, which when dry contains the pigment - the actual body of the paint).

Normally the binder resin remains dissolved in the solvent, while the pigment often settles to the bottom of the paint container and needs to be re-dispersed (by shaking or stirring) back into the binder before paint is applied.  But if you have shaken and stirred the paint and it is still clumpy, that means the binder has began to gel, and cannot be made liquid again. This is unrelated to the pigment that settles in the bottom of the container.

The way you described the problem seemed to me like the binder has gelled.  But if you can keep on shaking and stirring and the clumps get liquefied into a homogeneous liquid, then the paint is still usable.

[ewetwo]

Thank you Peter. I have a lot of Testors jars of paint and thought I was going to have to throw them out. You saved me. 

[peteski]

That will be a lot of energetic shaking or stirring.  Micro-Mark sells electric paint shakers, but I have seen threads here showing how to build a home-made paint shaker using Sawzall saw.

[ewetwo]

I bought one on eBay a year or so ago. It works well enough but not for enamels that started to gel. 

[peteski]

53 minutes ago, ewetwo said:

I bought one on eBay a year or so ago. It works well enough but not for enamels that started to gel. 

That's true, because no amount of shaking will revive those, but any non-gelled paint where the pigment just settled down can be made usable.