Hi julie,
I dont think that that adding sawdust to your paste will do anything except make it lumpy...
many add sawdust to pulp to add strength, then add the paste to it.
What are you using the paste on, pulp or strips?
I would just use a good glue diluted, like carpenters glue or weldbond but Ive never really had any problem with flour  - water - with a drop of bleach mix.. pieces ive made with it are still strong !