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Have heard of Dan Strebin, large collector? didn't know he conserved and backed posters.
1- can't imagine mounting a poster to anything other than a firm/stable surface; first to mount, then to restore any issues. Isn't "unprimed" canvas the same as "washed"? I may be confused/uninformed.
2- have never heard of anyone using a "cold vacuum press". sounds revolutionary! I wonder why more don't use it. Can only imagine a "vacuum press" is quite expensive.
@Charlie might be able to shed more light on this.
My approach since i'm early in my development, keep it simple.... and master the basics....