Glad to hear that it's progressing well, so far.
Cellulose thinners are pretty horrible and attack a lot of compounds. Normally they advise using nitryl gloves when handling them as they can attack lower grades of rubber (such as Marigolds), leaching out any plasticisers
Best "abrasive" for keying the surface is one of the non-woven fabric types, such as 3m Scotch sell - a relatively cheap version is sold by Toolstation as "Fit for the Job" sanding pads. Less risk of going through that ultra thin veneer surface with them
Yes it's nasty stff, although effective. It ate through my nitrile gloves eventually, so I tried washing up gloves for comparison (Robust by Aurelia, which proved the toughest I've seen so far - except this!). I think if I changed the nitriles often, it would be fine.
I'm fairly new to varnishing, so should I buy a particular brand, or even a spray on varnish or lacquer, if that's better? I'm hoping to avoid brush lines of course.
And should I sand between coats?
I'll begin by using "tacky cloth" to get rid of any surface dust, too.