I need to block some wood knots that bled through the finish of the door trim. The knots have been previously painted with latex paint and acrylic primer/sealer. It took just a month for the knots to become visible again. This time I am going with the shellac-based stain blocker (B-I-N ultimate stain blocker).
Here's what it says on the back of the can:
B-I-N is great for exterior spot priming and is the only primer that effectively seals knots and sap streaks. Note: prime only under high hiding paints. Otherwise, prime the whole surface with Zinsser Bulls Eye 1-2-3 after spot priming.
Also, the front of the can says Interior & Spot Exterior.
I am painting indoors, and the paint is white latex paint - probably not a high hiding one. What is the reason the label tells me to prime the whole surface - and what will happen if I spot prime? Do I need to apply a coat of regular primer over this shellac-base primer before painting it?