Knotty pine, blisters despite arduous prep ...

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I have a large garage door some of which is knotty pine.
Looked bloody awful when we moved in with the sap oozing out of it everywhere so I sanded it with 80 right back to wood where knots came through, then used 2 applications of Ronseal Colron knotting solution.
Then I spot primed with 2 goes of Zinser BIN Shellac primer.
Then painted 3 coats of Farrow Ball exterior eggshell. If I was being very critical I may not have waited the full 4 hours for recoat of the final 2 coats but I did a load on non-knotty pine at the same time and haven't had this issue anywhere else

The issue is huge blisters coming up, some literally the size of 50p coins. They are getting larger & larger! The door faces the sun.

I cut one open and it is the BIN primer that has lifted from the Colron painted surface - if I remove the blister the surface remaining is not the shellac but the wood.

The Colron knotting solution was about 5 years old (from an old job). Should I not have used it at all as apparently the shellac does block knots - I am at a loss as to why this happened. The shellac stuff is foul too! Need to work out how to fix now!
 
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Just seen the COlron stuff is indoor only! Oops... wondering if that & it facing the sun can't have helped
 
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Sometimes it helps to have a heat gun on the knots to make them ooze more, try and get some of it out. Some knots can run for years !! The Zinsser is probably the best thing you can use on this.
 
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thanks... the last paint they just bled through, but with this they are blistering - any ideas why this might be?
Can you give a bit more detail about the heat gun technique, how long do you do it for etc?
 
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Heat gun at close range to a decent knot will force the sap out. Its the sap from the knots that causes the paint to lift.

If there were only a few knots I'd suggest a film of 2 part filler over them to block the sap.
 

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