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Discussion Starter · #1 · (Edited)
Opinions? Appreciated in advance. This is an oddball. I installed new cedar siding for a homeowner, and he asked to make it look natural. I gave him a few examples, and he fell in love with cedar that was simply coated with shellac - we are talking premixed shellac in denaturated alcohol sold at HomeDepot. So, I did the job and all is good. Then the usual - the owner's wife calls and says, "We changed our minds and now want to paint it light gray." OK, I go back and get paid for the second time to do the job (so far so good): I apply Killz (original oil based primer) over shellac-stained cedar and then wait half a day and then paint Resillience over it. Resillience is a tough exterior acrylic latex paint from Sherwin Williams. Looks great so far. Now, two days and a little sun later, bubbles everywhere. And not just bubbles, but bubbles where on the inner side Killz peals off! So, there shellac is where it was (on the wood), while Killz oil based primer and the exterior paint are holding together and blowing bubbles. In other words, bubbles form where there is poor contact between shellac and the oil-based primer... which is bizarre because Killz stick to just about anything. I checked - there are no leaks that could have caused this, and the cedar I installed was brand new.

There is a pattern to the bubbles - some boards seem to be affected much more than others. Under the bubbles - wood looks normal - no water or condensation. Truly a brain freeze situation. Granted, we are in Houston, where it is constantly hot and humid, but this Spring it's not that hot. Humid and wet - yes, but this cedar stock was not more moist than anything I installed at any point in the last 20 years. Opinions? Appreciated in advance.

Questions:
1. Is there an oil-based product that's better than Killz?
2. Who said there is anything wrong with painting over shellac treated cedar?
3. How do I fix the job? Is it even possible without stripping everything?
 

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Ouch, you used two interior products on the outside? What could possibly go wrong. Maybe blame SW for the bubbling.

Didn't I see Houston got more rain in one day than an entire century or something just a few months ago? Hmmm I would say its a little wet on those boards. Did you check with a moisture meter?
 

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Discussion Starter · #8 ·
I assume the Kilz dried too fast, and didn't penetrate/bond to the shellac. FYI, Kilz, is called Kilz for a reason, it is made to "kill" stains. It is not a good bonding primer.
An old school slow drying long oil would have worked much better.
I confess I've used (and likely overused Killz). The original Killz - in my defense - says "for interior and exterior use" on the can. The issue we are not mentioning here is this: painting cedar means possibly getting stains from knots... unless you use a strong antistain agent. Agree?
 

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Discussion Starter · #9 ·
Shellac is for spot priming exterior usage only. The bubbles could be a sign of trapped moisture or solvents escaping the oil primer.
I was tempted to think this... but shellac separated from the bubble (the bubble is made of kills and the paint, and shellac is intact on the wood. Still, you're probably right about the moisture IMO.
 

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Discussion Starter · #10 ·
Ouch, you used two interior products on the outside? What could possibly go wrong. Maybe blame SW for the bubbling.

Didn't I see Houston got more rain in one day than an entire century or something just a few months ago? Hmmm I would say its a little wet on those boards. Did you check with a moisture meter?
One good reason to hate H-town is the weather. It makes all exterior work harder... I'd even say painful. Granted, shellac may or may not belong outside, but the original Killz is clearly labeled as "interior/exterior", and, as I may have mentioned in a reply to someone else here, it is pretty important to use " something" under the paint to ensure that cedar knots don't come through the paint and don't become stains. What's your opinion on preventing cedar stains? (with or without shellac)
 

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One good reason to hate H-town is the weather. It makes all exterior work harder... I'd even say painful. Granted, shellac may or may not belong outside, but the original Killz is clearly labeled as "interior/exterior", and, as I may have mentioned in a reply to someone else here, it is pretty important to use " something" under the paint to ensure that cedar knots don't come through the paint and don't become stains. What's your opinion on preventing cedar stains? (with or without shellac)
I don't trust any product on an exterior that says interior/exterior
 

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One good reason to hate H-town is the weather. It makes all exterior work harder... I'd even say painful. Granted, shellac may or may not belong outside, but the original Killz is clearly labeled as "interior/exterior", and, as I may have mentioned in a reply to someone else here, it is pretty important to use " something" under the paint to ensure that cedar knots don't come through the paint and don't become stains. What's your opinion on preventing cedar stains? (with or without shellac)


There are several lines of Kilz of course, and we don't know which one the OP used.

As far as the original Kilz primer though, I always though it was interior only. As per this spec sheet;

http://www.kilz.com/MCContent/MC_Product/KILZ_CA/KILZ_CE_TDS/Original_CE_TDS.pdf

Slow drying (long oil) ext oil primers are what I've always thought was idea to stop tannin bleed on woods like cedar. Short oils suited for ext use, like Coverstain, can work also but don't tend to hold up as well in the long term due to lessened penetration.


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Discussion Starter · #13 · (Edited)
JourneyManPainter - This is a good point. I never thought about this. I guess it's like, when someone says, "we can sell you any kind of car as long as you sign right here"... it probably means that they just really want to sell you a car. On a related point: I noticed that Killz isn't as popular in some states; I see less of it in the Northeast and in California, for instance. But in Texas very few jobs go without Killz. Its side benefit is that it kill(z) and prevents mold, which in Texas is like scorching sun and rain. That said, Killz here is like a panacea, historically speaking. Where are you located? (did you say Vancouver? No problems there with mold I bet) What's your exposure to the Original (oil-based) Killz (in the white gallon can) professionally? And most importantly, what would you use locally as the go-to product for your "hardcore priming" jobs?
 

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Aside from being brittle, the main reason shellac isn't suited for exterior use is it forms a vapor barrier. Unlike regular exterior acrylic or oil primers, a shellac film is non permeable. This is the quality that makes it so good for sealing in smoke, urine, or other residues on interior surfaces. Vapor cannot penetrate it.

Conversely, this quality makes it a bad choice for exterior priming where the substrate must inevitably absorb and release moisture vapor as atmospheric conditions change. Spot priming is ok because your not creating a continuos non permeable film over the surface and moisture can still escape around the sealed spots.

A full shellac prime on an exterior wood surface can be disastrous. I've experienced this myself many years ago, and recounted it here in the past.


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Discussion Starter · #15 ·
Sounds like bulls eye

Jmayspaint - it seems that you have experienced a more-or-less the same scenario as mine. I obviously didn't use shellac as a primer... but it ended up being exactly that - a primer that was coated with Killz first and with a non-oil paint second. That said, your vapor barrier reference is convincing, and it appears that this is exactly what I have suffered: I had stock fully covered with shellac, and then I painted over it..,,,, that said, Killz or no Killz, it can act weird... however, let me ask you. Let's say you were approached by a customer who said "I have a wall covered with cedar that was treated with shellac... can you please paint it for me using this exterior moisture-barrier Resillience paint from Sherwin Williams?" And if not, what would you recommend to change the appearance of this wall to a painted wall... apart from reinstalling new cedar siding stock?
 
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