I'd say sand and second thin coat of primer. Why not use an undercoat?
It's "Sandtex 10 Year Primer Undercoat" which even with lots of stirring keeps trying to revert to bottom half sludge top half watery in the tin. (In general what's the difference?)
@hoefla started
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I'd say sand and second thin coat of primer. Why not use an undercoat?