I've been making Ziatype prints for a while now. If I print 8x10 or smaller, my negs lay pretty flat. When I make prints that are 8x13, I get patches that are blurry due to the digital neg having full contact on the paper. The coated paper buckles on area coated with an emulsion. Do I need to use a vacuum frame for making larger prints or is here a method of getting by without one? I use DIY 16x20 printing frame which works OK. Any insight would be great.