Gum over Cyanotypes

[Loris Medici <mail@loris.medici.name>]

I did few gum over cyanotypes recently and I happen to like them much. See
the most recent one below here: http://dwarfurl.com/008fa

The print is on HCl acidified (2 minutes in 2.5%) Fabriano Artistico EW.
One -1/3 stop exposed cyanotype layer + 3 gum layers. (1. PR206 5% AD +1/3
stop exposure, 2. PV19 Rose + PBk9 5% AD normal exposure, 3. PBk9 2% AD
+1/3 stop exposure.)

I print the Cyanotype on unsized paper, then size with 3% gelatin. After
sizing, the paper change dimensions and you can't register the negative
perfectly on the first gum pass - 2mm larger in both horizontal and
vertical orientations (print size 10x6.6"), the registration gets perfect
only in the second or third gum pass. (See the resulting blur at the right
edge in the middle.)

My question is: How do you manage to get perfect registration for gum over
layers?

Any tips and tricks would be highly appreciated.

Regards,
Loris.

P.S. I have read Lukas Werth's method somewhere else: Affix the paper on
dimensionally stable support - such as an aluminum sheet - with gelatin,
print, varnish the print (acrylic binder + mineral spirit) to "protect"
the gum layer and then put the print in warm water (this is where you need
"protection"!) to melt the gelatin and release the print from the support.
I would like to hear about any suitable varnish that can be used for this
purpose too...