All these blocks were the slower-working Great Art brown lino. Each about 10 cm square. Left to bite for two hours. I tried to put the caustic soda on as thickly and evenly as possible. (I think the thickness of the soda also affect how much it bites, but that's a future experiment.)
Stopping out varnish: Works, but needs to go on thicker, and leave it to dry longer. This had dried for about one hour.
Wax crayon. You can just about make out some curved lines towards the left of the print, but again, it needs to go on more thickly.
Sellotape: Works well, if you press it down hard enough. This block had several strips of tape across it, which I took off at intervals. The dark stripe at the bottom was not bitten at all; the next strip for about 30 minutes; the next for 60 minutes and so on.
Sellotape: Twisted and folded to make patterns.
Sellotape and wax crayon
Will power: Just applied the soda on the block in selected places. This doesn't work too well if you do what I did: drop the block so that the soda ended up over nearly all of the block.