Just get some washes made by Flory Models. Those are all water/clay-based and you remove the excess with sponges/q-tips/brushes dampened with water. 100% safe for all finishes. The stuff is an absolute joy to use. You also don’t have to worry about getting it precisely in each of your panel lines, you just slop the stuff on the whole model with a big brush, wait 30 minutes, and then mop up the excess until you get the look you want.