Sap Bleeding Through Primer on Ceiling Boards
Advice on stain-blocking primer, on cleaning up sap, and on proper wood drying. January 14, 2009
Question
I have pine boards the have been installed on an interior ceiling but not before being primed with Sherwin-Williams Prep Right Pro Block stain sealer Oil Primer.
After being installed onto the ceiling I went to the job to do the usual prep before needing to apply the second coat of oil primer. I noticed in several places that sap has bled through the primer and is pooling.
My question is, how is sap removed and do I need a different primer/sealer to block this sap from coming through?
Forum Responses
(Finishing Forum)
From the original questioner:
I have one answer from the contractor that installed the pine boards. He said as the boards were stacked after primer was installed, the sap leaked from the bottom un-primed sides of the boards onto the primed sides. So now my question narrows down to what washes sap off so I can apply the second coat of primer?
From contributor A:
Oil based primer has no place inside a house. BIN white shellac primer is the only primer needed besides a decent waterborne acrylic. Shellac provides the most effective stain blocking especially with knotty pine.
From contributor W:
An organic solvent like turps or paint thinner should work. However you might also remove the primer. I used Kilz which is an oil based primer and noticed that I could remove it with some paint thinner on the wood after it had dried. Some oil based products will polymerize after application but some like the fast drying ones like Kilz seem to be just some pigment and binder in an organic solvent.
From contributor C:
Pine-Sol may work.
From contributor R:
Shellac works best for a primer on pine knots. If it is dripping or pooling, the pitch wasn't set properly in the lumber. You are not going to stop it with any primer sealer.
From contributor S:
If you can find the product brand "Lestoil" cleaner, it has the highest percentage of pine oil of all the ready-made cleaners I have tried. Like dissolves like. I have found it at hardware stores and some grocery stores too.
From Gene Wengert, forum technical advisor:
In lumber drying, we use a temperature over 160 F to evaporate the sap or resin that is liquid at room temperature. I prefer 180 F. Your wood was not treated in this manner and as it is intend for interior use, I would argue that the wood sold to use was not able to perform as the product must perform, so it should be returned and the supplier should cover the cost of replacement. A lawyer can help you with this.
If you choose not to do this, then you might consider supplying a lot of heat (in place) to evaporate some of the resin or pitch. But the heat will also dry the wood and cause shrinkage, so the end results may be unsatisfactory. Remove the paneling and then heat it and then reinstall it. Perhaps get a lumber kiln to do the heating for you.
As mentioned, shellac is a fairly good (but not perfect) sealer. I also agree that an oil-based primer is not appropriate on the inside.
