I'm repainting an old plaster ceiling in my house. I took off the popcorn texture, scraped, and sanded everything, which exposed a large stain that looked like a water stain.
I soon discovered about 6 inches of bat poop (guano) directly above the stain in the attic by the chimney, so I removed that. So, the stain is either from the guano itself, or the chimney area happened to be conducive to both bats and water leakage.
Anyway, I painted the ceiling with water based primer, and applied a few coats to the stained area. Then I used white latex ceiling paint, and the stain still bled through.
I've read other posts that say to use oil-based primer to cover a stain, so my question now is; can I put oi-based primer over the latex primer and white latex ceiling paint? Or, do I have to sand back down to the plaster in order to apply the oil-based primer?
I would use Kilz-2 over the latex ceiling paint and then reapply your latex paint over that.
When something "bleeds through" a coat of primer or paint, what's actually happening is that something is dissolving in the water or mineral spirits in the primer or paint and diffusing through the wet film. So, for example, something that's soluble in water (like Easter Egg Dye) will bleed through a latex primer, but probably not an oil based primer. Similarily, something like duct tape glue will bleed through an oil based primer, buy probably not a latex primer (because the glue dissolves in mineral spirits).
So, using a latex primer to try to hide something like bat guano (that's likely soluble in water) wasn't a good idea from the get go. You'd have been better off using an oil based primer.
The biggest gun in the arsenal when it comes to hiding stains is a shellac based primer like Zinsser's BIN primer. Shellac is only soluble in alcohol, and so once the shellac dries it forms a physical barrier over the bat guano. Painting over the shellac layer with either a latex or oil based paint won't soften the shellac at all, so the bat guano shouldn't be an issue once it's behind a layer of dry shellac. You often hear of Zinsser's Bin primer described as a "white pigmented shellac", which is just a fancy way of saying shellac with titanium dioxide pigment in it to make it white in colour.
I can't comment on KILZ-2 as I've never used the product.
However, you don't have to remove anything. Simply paint over what you have with a primer that the guano won't dissolve in (and I'd be surprised if it dissolved in either mineral spirits or alcohol) and then paint over that stain killing primer (when dry) with whatever paint you want.
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