Which Dillutant Won't Attack Enamel Finish?

Actually, some turpentines will attack some acrylic finishes.

You can use water or alcohol on enamel finishes without affecting them. This pretty much limits  you to an acrylic system for washes and similar weathering techniques.

On very thoroughly cured enamels, you can try using mineral spirits, providing you don't leave it too long and don't touch the surface for 12 to 24 hours after application. The problem is that mineral spirits, although the least aggessive of the enamel solvents, is also essentially universal.

Another option is to coat the enamel with several light coats of Future. Once the Future has formed a continuous barrier, you can use enamels or oil paints for weathering and detailing, since it is essentially impervious to mineral spirits. 

Isopropyl Alcohol will work quite well as a solvent and should not attack your enamel finishes.

Of course, you're always safe with plain water. 

I've even had success using mineral spirit thinners as a wash medium, but in order to do so the paint needs to be fully cured.  I've used it successfully with both oil and enamel washes.

Stay away from ketones (acetone, MEK, etc.), lacquer thinners, the Testor's airbrush thinner in the black can (it's too "hot" IMHO), and you should also be careful using methyl or ethyl alcohols (both are present in denatured alcohol).

Just my opinion. Hope this helps.

Gip

Methyl and ethyl alcohols are also present in some turpentines. While alcohols generally do not affect fully cured enamels, there are such things as "enamel lacquers"—essentially an enamel paint that contains a lacquer component. These are becoming more common in hobby paints. The lacquer component is often soluble in alcohols.

I, too, have had success using mineral spirit washes over enamels, as I implied. But my paint jobs have always been very thoroughly cured before any weathering was done. I've even used acrylic washes over acrylic, but it is not my preferred method!

I'm asking the question because I will be using Mig dirt/earth pigments to dust my tank models. Those pigments can be applied with water. I'll settle for water. In my experience, confirmed by your comments, "cured" enamel finish is extraordinarily vulnerable to anything other than water. Gentle touching with mineral spirits to create dark washes is fine but nothing more aggressive.