Why don't you use the traditional destaining solution for Coomassie-stained gels? It is very similar to the "fix" used for pre-treating gels that will get silver-stained. Coomassie destaining solution: http://cshprotocols.cshlp.org/content/2007/2/pdb.rec10717.full?text_only=true. Silver Staining protocol: http://proteomics.facilities.northwestern.edu/items-2/silver-stain-protocol/.

 this way (Coomassie destaining solution) could be accelerated with the help of heating of microwave,but dong use it many times,or the gel would become deformed.