Nah, dry retarder can be hazardous stuff. 1 drop too many and your wash will NEVER DRY! (*evil laugh here*)
If you want to keep with acrylics, mix some flow-aid with distilled water (10:1 water to flo aid) and thin your paint down to the appropriate wash consistency.
Or
Gloss coat your mini. Thin the acrylic paint down with straight rubbing alcohol. Works quite similarly to an oil based liner wash.
But...
What I would advise would be to drop the acrylic paint altogether, and do something pretty similar.
Go to Michaels/Joannes/similar arts crafts shop and take a look at their inks. besides the typical sepia, black, purple, their are lighter colors. Yellow, Orange, Red, etc.
Then, follow these directions:
http://www.awesomepaintjob.com/index.cfm/resources.recipes
---2 cents given, good luck