White spirits. It doesn't harm the resin. But it will remove the mould release agents. I have used it on
FW models, MicroArtStudio resin bases and some resin pieces I bought on Ebay. (But I haven't tried on finecast) Its worked on everything I have tried it on.
Simply dip the pieces in the Whitespirits for a few seconds (I have even simply dipped an old brush in the whitespirit and brushed it on and its worked)
Then rinse it under the tap for a few seconds and allow to dry. (You can brush if you wish, but I feel its un nescesary in most cases)
And Citadel paints WILL stick after this. I have even half dipped a resin base in the whitespirit once, then rinsed with water and then painted. The paint stuck perfectly to the dipped side, and not at all on the non dipped side.
Nathanael