Re: shrinking Masa
I won't enter in the Masa problem, but simply give my opinion on shrinking.My understanding of shrinking is to let the paper absorb as much water as it can (in other words saturate it) and then dry it. I don't think that any paper would benefit a soak more than 30 minutes unless it's extremely heavy (such as > 282lbs / 600gsm) and / or is heavily sized. BTW, I don't understand why people pre-shrink in hot water and/or for hours -> that would be harmful to the original sizing!? Of course I may be completely wrong here -> I'm sure someone will correct me and/or explain why they're doing so...
I compare the shrinking of the paper to what may happen with new cloths. In fact, shirts, socks, sheets, curtains, etc. are used to become shorter after the first washing if this has not been performed by the manufacturer. In this way, also the paper, made from a wet process but almost ever calendered, needs to be wetted again to allow its fibers to find their best position. If this is performed in warm water the sizing does not hold the fibers any more, and so it is more effective. The drawback is that the sizing is damaged and you need to size again.
However, I believe that the gum layer itself contributes to shrink the paper, making this problem not ever (or not fully) dependent on the pre-srinking and unsolvable if not glueing the paper to a rigid support. Try drawing lines near the sides of a pre-shrinked paper, cover only one half with gum+dichromate, expose, develop and dry, then measure the distances between the parallel lines comparing the zones with and without the gum layer...