I have just gathered several pick up truck loads of rocks rescued from a construction site nearby where they had to blast through a lot of rock. I am using them to outline my flower beds in a full sun location. The rocks are now dirty red from the Georgia red clay they were buried in. How can I get them to turn gray and look old and lichen covered like they've been there for years? I can't use the buttermilk method because there is full sun at this site and moss won't grow. What else can I do? I tried to hose off the red clay, but they're still stained red. Thanks.