I've never used Mr. Dissolved Putty, but I have used Mr. Surfacer to fill small gaps.

I'd recommend Tamiya putty to fill medium gaps and Mr. Surfacer to fill small gaps.  Mr. Surfacer can also be used as a primer.

What primer did you use?  Did you wash the plastic before applying it?  How long has it had to dry?  With the issue you're having I'm thinking that you may have used an acrylic primer.  One of the biggest downsides of acrylics is that they don't feather when sanded and you end up having the issue that you appear to be having.