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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?

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I've used BIN primer with great success in the past. The reason the can says to use under a high-hiding paint or to prime the whole surface is so the primer doesn't show through.

Say you have some unpainted, knotty wood and you spot prime the knots. When the whole board is painted, the spots where the primer is will be very obvious since they are bright white and absorb the paint differently.

I've used BIN to prime an entire wall of knotty pine, smoke stained paneling and then painted. It worked well since the whole surface was evenly primed before I used the paint.

For what it's worth, you wouldn't want to spot-prime over normal stains on an old with a water-based primer before painting either. You'd prime the whole wall or you would have an inconsistent sheen at least, and visibly lighter spots in many cases.

Looking back at your question, since you are going to be spot-priming a previously white-painted surface and you're going back over it with the same, white paint, I think spot priming with BIN should be OK. Just be generous with the spots so the stain doesn't travel under and around the BIN spots.

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  • Excellent answer. You can also use a regular can of spray shellac in the place of BIN if you do not have it on hand. A good coat of high hiding primer is key to having a smooth consistent surface. Since this is for wood trim, an even coat of primer can be easily applied with a small roller meant for smooth surfaces. You wouldn't want to use a textured roller in this case. – Jason Hutchinson Nov 23 '15 at 20:29
  • I see, so because the paint is absorbed differently, there will be differences in sheen and shade. But in my case the trim to be spot primed already has a layer of primer and paint. Is it ok to spot prime then? – user443854 Nov 23 '15 at 20:32
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    Edited my answer as you typed your comment. I think it should be OK in your case, and it's what I would do, but try on one board before doing all of it to make sure the sheen isn't noticeably different over the spot-priming. – JPhi1618 Nov 23 '15 at 20:50
  • Confirmed experimentally: spot priming previously painted surface works just fine. There is no difference in sheen at all. Three weeks later, BIN is holding solid, no hint of the knots bleeding through. – user443854 Dec 16 '15 at 18:53
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You can also spot prime with PVA wood glue. Works great and dries quick. However, it will be a smoother finish where the wood glue goes on. That means your topcoat will have shinier spots in it.

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