Thanks Derek. The reason I asked is that all recent examples I've seen being demonstrated which involved simple mixing, and without a settling period or sieving/filtering stage, have been very obviously gritty (not that the people demoing seemed bothered!) and as one source puts it:
The next japanning mixture is referred to as “Cold-cure” japanning. Consisting of spar varnish and asphaltum, this mixture is simple to prepare and provides a very deep black, glossy finish closely matching original japanning.
It is not a quick process, requiring days or weeks to mix and settle.