Today was a fair and warm day, so I took off in mid afternoon from my paying work to apply the third coat of varnish. I sanded again with 400 and 600 grit paper, then a final pass with a plastic scrubber pad. I also added more black oil paint and thoroughly mixed the varnish. The colour is now noticeably darkening.
I had planned to make the Sinister a pale grey, but it may come out quite dark as well. Maybe I'll use a clear varnish for the last few coats.