Greasy marks that won't erase...

1. Hi, I like to work over lots of layers and my most used tools are the just add water blender and also the soften effect. When you soften and blend with water, there is often a halo that surrounds the image, and I just go around with the eraser to neaten things up. However, on some colours.. black, greys, browns especially...it leaves a greasy stain around the image and the eraser tool just moves the 'grease' around.

Another issue I'm having is that the image looks great as a RIFF, but once I change it to a jpg for printing, there are often weird marks all over the image as if I've scribbled on it with a felt tip pen. These can't been seen when working on the RIFF, but once the image is saved as a jpg (or in fact any other file format that can be opened elsewhere and used for printing) all these squiggles appear...it's very frustrating.

I work at a resolution of 500ppi and other than these two issues find essentials is great, but the greasy stain and squiggly line thing is driving me mad!! 

What am I doing wrong? 

Thanks , Shubun