I’m having a bit of difficulty finishing exterior WRC. The house is 4 years old and is being stripped and repainted for the third time now due to blistering paint. Exterior construction materials include mixed species, the vertical siding being clear vertical grain WRC, and the facia and soffits being clear vertical grain WRC as well. The window and doors are FEQ genuine mahogany, and the balance of exterior trim/millwork is Sapele. All the above were painted according to architectural specifications which included an oil primer and two finish coats of B&M Aura exterior low luster paint. The color is Kendall Charcoal, being very dark, with a light reflectance value of 13. The surfaces get extremely hot in direct sunlight due to the dark color. The paint is fine on both the Sapele & Mahogany, but is blistering on the cedar, and only on the south elevations. The cedar was primed all sides, as well as every end-grain cut being primed during assembly. I took numerous moisture content readings and none are greater than 6% so I’ve ruled out moisture. The blistering is isolated to individual boards, the blisters occurring across the entire lengths and widths of boards. 10% -20% of the boards are experiencing finish failure. Being that the blistering is isolated to individual boards I’m ruling out solvent blisters as well. Where one board failed, the adjoining board is fine. The primer is releasing from the cedar only when top coated, or on locations where a second coat of primer is necessary. Both wet and dry film thickness are spot on to manufacturer’s specs. There is no visible evidence of moisture or solvent within the blisters, however, what I did notice was a sticky pitch or sap-like residue beneath the blisters. This substance appears to be exuding through the oil based primer, actually causing the primer to darken and turn gummy. The exuding substance doesn’t have any dimension to it, and is more like a sticky film. I’ve included a picture to illustrate, the arrow in the photo illustrates a bleeding board and the effect it has on the color of the primer after being intercoat sanded, and ready for finish. I’ve also included a photo of the stripped siding, thinking the coloring of the cedar was a bit odd, many of the boards having the appearance of being sapwood, but at 4” wide/vertical grain, I don’t think sapwood rarely exceeds 45mm. I’m suspecting that the cedar is immature new growth. Finish failure is occurring randomly without any correlation to a particular board color. Are there essential oils, sap, or pitch in newer or quick growth cedar that could be volatizing due to the heat and solvent based primer? The cedar was stripped mechanically and orbital sanded to #120 to provide plenty of tooth. It was then vacuumed and wiped down with acetone. No boards were left exposed to the elements and were primed within hours after initial stripping. There was no visible evidence of an extractive of sorts before priming. I’m hoping someone here might be able to share some knowledge as to the chemical properties of Western Red Cedar that might be causing this, and provide a possible solution rather that a tear-off/re-side and re-trim which the client is willing to do as a last resort. A breathable finish such as an acrylic semi-solid stain is not an option due to the variegation in color of the boards.
The primers used on all three re-paints were B&M 100, 094, and 366. The 366 is the same as 094, yet comes in a deep base to allow it to be tinted to the finish color without compromising the performance of the primer. All primers were tinted to the finish color.
I’m really at a loss on this!

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This is a good one for Gene Wengert to look at. He is the guru of wood knowledge.
Also he has a forum here on Woodweb.
I believe it is under Kiln Drying

If the blistering is due to sap bleed through you could try spot priming with BIN shellac based primer to seal it in.

In your case, without being there, I can only guess, but I think I know the answer. When an old growth tree is growing, there is an anaerobic bacteria that often gets into the tree. It is in small compartments running vertically in the tree and in the bottom log. While alive, it creates fatty acids that turn rancid with time. Did you or the homeowner mention any funny smell? The bacteria also create some slime that tends to plug the moisture movement. The bacteria also somehow create very high moistures in the tree, as they are anaerobic. Finally, the bacteria create enzymes like pectinase that destroy some of the wood’s strength. I have seen discoloration in plastic overlaid molding...it comes through the plastic. The infected area is sometimes called wetwood or sinker stock. It often has a darker color.

Thanks for the responses. Doctor Gene I think nailed it. After initially posting this I noticed making an error, the boards appear to be old/slow growth, not new growth. I needed to correct myself after counting growth rings. The prior painter appeared to have masked off problematic boards, coating them with Bin, but they still popped. I did notice the ring separation on the right board and the dark vertical streaks on others which are indicative of bacterial wetwood. The fatty acids would account for the solvent soluble bleed, fatty acids being an oil. Thinking the mineral spirts and heat activated the slime. That would also account for the primer turning gummy and not drying for weeks. It all makes sense now. Thanks again.

FWIW: I was told that 'Bin' shellac is not to be used on any exterior wood product. Correct me if I am wrong. I did try using it to fix a chronic 'sap' problem on a painted pine exterior door, and it always came back within one season.....oozing all the time.

Each year, for 5 years I painted that door!

I think you have to replace the board!

I wrote this on one of the other threads last week. BIN is an exterior spot primer. What area a spot covers I have no idea.

Shellac used to be used as a glue on wooden boats. BIN is rated as a spot exterior primer. I primed some cedar flower boxes I built inside and out. They were on my own house for about 5 years full of flowers. dirt and water. My wife was supposed to topcoat, but never got around to it(how surprising). Zero peeling inside or out. BIN is an excellent product. I honestly do not know of another product that would have performed in those extreme conditions. I did it when I had no time and little kids. Interesting accidental experiment.

The bleeding or oozing of sap or pitch in pine is a different issue than the WRC issue. The pitch is made up of many different chemicals, some of which are liquid at room temperature. When drying pine, it is recommended that the wood be heated 160 to 180 F. This evaporates the chemicals that are liquid at 100 F or cooler The sap left in the wood can be softened with heat or rarely some chemicals in finishes. Every piece of pine and most other softwoods like WRC have a potential sap issue so needs to be heat treated., which is common in kiln drying indeed,

The bacterial issues I mentioned in an earlier post are a totally different. Hemical. are not too common, and. Annoy be stopped with heat or various chemical coatings. I have seen the bzcterial chemicals also move through plastic overlays. I have seen it move through gymnasium flooring finishes. The foul odor is not fixable with a finish but requires total replacement. Be happy that few of you have encountered the bacterial issue, as such wood almost always has shake and would be eliminated in processing.

Correction:
The bleeding or oozing of sap or pitch in pine is a different issue than the WRC issue. The pitch is made up of many different chemicals, some of which are liquid at room temperature. When drying pine, it is recommended that the wood be heated 160 to 180 F. This evaporates the chemicals that are liquid at 100 F or cooler The sap left in the wood after het treating can be softened with heat (including heat from sanding or a finishing oven) or rarely by some chemicals in finishes. Every piece of pine and most other softwoods like WRC have a potential sap issue so needs to be heat treated., which is common in kiln drying indeed,

The bacterial issues I mentioned above in this post are caused by a totally different. chemical. are not too common, and. cannot be stopped with heat or various chemical coatings. I have even seen the bacterial chemicals also move through plastic overlays. I have seen them move through gymnasium flooring finishes. The foul odor is not fixable with a finish but requires total replacement. Be happy that few of you have encountered the bacterial issue. Most bacterial infected wood almost always has shake and would be eliminated in processing.

Correction:
I incorrectly stated that fatty acids are oils, which they aren’t. My conclusion that the solvent in the primer was activating the bleed is likely questionable. What I’ve gathered, as Gene mentioned, the fatty acids plug up the water transport system in isolated regions of the tree, some being pencil thin, and moisture is trapped under extreme pressure which many times won’t escape during the kiln drying process. When checking moisture contents as I did, there is a strong likelihood that unless I probed those exact locations where pockets of moisture were trapped, that I might be getting low readings. I noticed that many of the problematic boards have a bright yellow to orange discoloration. The staining of the primer starts as a small dark wet looking spot, and just keeps growing, turning entire board faces dark and gummy. I’ve never seen this condition before.
I’ve been seeing a lot of white oak sourced from wetwood infested trees, with flooring and veneers blowing apart during processing and finishing, including ring separation across entire rift-sawn veneers.