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I recently purchased an old house. Due to its age, I don't really know the specifics of the materials used; things like type of paint, composition of drywall, etc. There have also been some animal intrusions over the years behind the drywall (the known intrusions have been fixed).

I have been kicking around the idea of using Shellac Primer on all of the interior walls and ceilings of the house before painting. The idea is that it would form a barrier against the unknown ingredients of the original paint and drywall as well as any unknown animal secretions behind the drywall.

Is this a misguided idea?

I've never used Shellac Primer. Some concerns I have with using it are

  1. The amount of VOCs from the primer after it is dry; I just don't know if this is considered safe for health to use so liberally in a living space?
  2. The reduced breathability of walls and ceilings having unintended consqeuences such as mold?
  3. Whether this is even a correct use for Shellac Primer?

I appreciate any help.

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    "VOCs from the primer after it is dry" Should be approximately none, having dried...
    – Ecnerwal
    May 22, 2023 at 17:56

4 Answers 4

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I don’t see any downside. I’ve been using shellac based primer for many years, and I think it’s one of the greatest problem solvers ever. I wouldn’t hesitate to use it on any indoor surface that’s listed on the label. It will improve the coverage and smoothness of the final coat.

As far as I can tell with my nose, once the alcohol evaporates and the primer has fully dried in an hour or so, there are no more VOCs at all. There’s certainly no “curing paint” odor.

Breathability of walls? No idea, but I’ve never had a problem in that regard.

Just be aware that shellac based primer has a limited shelf life. Buy it from fresh stock and don’t buy much more than you need, hoping to use it next time you paint. The last time I looked, they wanted you to use it up within six months of opening the can.

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  1. Shellac itself is as far as I know pretty safe. It's made from insect secretions. The strong smell from shellac finishes is the solvent. Primers might be different but the amount of VOCs should be in the spec sheet.

  2. Walls (at least in cold climates) should have vapor barriers incorporated into them on the "warm" side so they shouldn't have to breathe on the interior side. If you have enough moisture in the walls that it has to actually dry to the interior you're going to have problems shellac paint or not. I think an old house with plaster walls and 6 layers of oil and latex paint probably doesn't have the walls breathing much anyway.

  3. I don't think shellac primer/sealer is meant to contain active problems. If a leak stains your ceiling and you fix the leak then want to paint over the stain without it showing through the paint, you cover it up with a sealing primer. If you still have the leak, no shellac primer is going to stop it. It's meant to create a solid surface for application of new paint. It won't help if you have asbestos in your ceiling or lead paint: it'll stay on top but the way you disturb the asbestos is knock holes in the wall and, again, primer isn't going to stop that. It may help if your house smells like cigarettes.

So if you want a new start for paint, use the sealing primer. If you're trying to seal away some eldritch evil, shellac won't help. If you're new to the house and you're worried about it, you may want to live in it as is for a while and look for problems before you apply broad solutions like painting every wall with shellac. If you have leaks or animals or moisture issues, you'll just have to fix them and repaint.

Hopefully people will correct me if I'm saying something obviously wrong.

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    One of the more common uses for shellac is as a coating for pills - so yeah, shellac by itself is literally edible.
    – Ecnerwal
    May 22, 2023 at 21:21
  • The solvent used for shellac is alcohol. Not necessarily pure ethanol, but I'd be a bit surprised if it presented a longterm VOC issue. It's just too V an OC at STP to stay around for long.
    – keshlam
    May 22, 2023 at 22:07
  • The primary use for shellac primer/sealer (a shelf-stable form of dewaxed shellac) is when you want to use an oil-based finish over an existing water-based finish, or vice versa. Shellac, being alcohol-based, sticks to both and both stick to it. Of course shellac can also itself be used as a transparent finish, though like oil it will add some color (usually in the yellow-to-red range); it isn't very durable, but it's trivial to refresh if/when it does get damaged.
    – keshlam
    May 22, 2023 at 22:10
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    +1 for "you may want to live in it as is for a while and look for problems before you apply broad solutions like painting every wall with shellac. If you have leaks or animals or moisture issues, you'll just have to fix them and repaint."
    – Bear
    May 23, 2023 at 13:46
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There's nothing wrong with using a shellac or product like BIN for your entire interior. In fact it's an excellent coating for coverage, bleed, and sandability. There may be other products that offer better adhesion or are cheaper, depending on your need.

One important thing to keep in mind is how you apply it. Some shellac primers are alcohol-borne and when you apply it you will be exposed to alcohol vapors. Pros who work with these products regularly often use full face respirators.

Did you know you can get inebriated by absorption of that vapors through your eyes?

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I would like to answer specifically your second question:

  1. The reduced breathability of walls and ceilings having unintended consqeuences such as mold?

This is absolutely something to keep in mind. From what I have been able to find online, BIN and similar shellac products have very low vapor permeability -- around 0.4 perms, but you should try and find the number for the actual product you purchased. A low number means the coating doesn't "breath".

This means a continuous, well-applied coating of shellac would act as a vapor barrier. If your walls already have another vapor barrier included in the assembly, you could indeed run into moisture problems including mold. Any water that found its way in between the two vapor barriers would not be able to dry to either side (and water always finds its way inside).

Whether this is a problem in practice depends heavily on your house's site conditions and what is in the existing wall. A stucco house in rainy Florida almost certainly would see problems with an impermeable interior wall covering. Since you say your house is old (and therefore probably also air-leaky and poorly insulated), you may actually be less likely to have problems because all that "leaking" energy moving around promotes drying.

There is an organization called BSI that hosts a free database of technical guidelines and lessons-learned about building science, with a heavy focus on moisture dynamics (I am not affiliated, I just have found it to be an excellent resource). If you live in the US or Canada, they've likely helped shape your local building codes. I would encourage you to take a look -- Joe Lstiburek is an engaging writer even for the layperson:

https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

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