I was always double coating before I started using gold toner, I didn't bothered afterwards. (I'm sure I could still get a better dmax by double coating, but the blacks I could get with gold toned single coated prints was convincing...) About the horrible results after filtration: That's definitely contamination, what filter paper did you used? I use "Melitta" brand coffe filters, they're completely safe. (So safe that I recently filtered USD 800 worth Pd solution!!!) And definitely use very clean vessels and tools while working with the solution. Strange stain patterns in the print usually indicates problem with the brush, not the solution BTW...