Oops! Sorry, I should have said "thickening medium" not thickening paste.. The brand I have is Talens and it is designed to increase the viscosity and decrease the gloss of paint. I hope nobody's rushed in and had a disaster using paste. Eeek! That's what blogging with a tired brain does for you.
Thank you all for your comments yesterday - I feel humble that ladies with such experience and skill are learning from me! There were one or two points I could/should have mentioned:
1. You can paint the assembled printing block with pva, gel medium or gesso before you ink it up and this will change the textures but unify the surface, also the ink/paint will not be absorbed by the materials you are using.
2. The ideal paper to use for printing is paper without size - that is, absorbent paper like newsprint or sugar paper. Proper printing paper has no size. In my opinion you can experiment on many surfaces and see which results you prefer. I am sure that working in mixed media no approach is wrong - it's just what works for you.
3. A word about the thickening medium - it is horrible mixed with water, it goes all slimy, yuck! Clean up your printing plate by scraping with a palette knife and wiping off the residue with paper and finally using a damp cloth.
This picture shows one of my blocks "inked" up and the resulting print. I made the bluey coloured print first and then overprinted when it was dry in red. You can also mask off part of your print that you may not want to lose and overprint again.
And finally, from my sketchbook, this is my original failed attempt at collagraph. I like the textures on the plate so I will have another go using these techniques and see what I get. In this attempt I simple used paint without any additive and because the surface was uneven it only put the paint down where it touched the paper. Better luck next time I hope.