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If they are well made you'll still have to treat the knots and they will possibly cause early failure of the paint if they weep sap. One way to sort is infra red which melts out the sap, then filling.
It's just the way it is and the reason most joinery grade work is in a wood with no knots. It's normal for this type of door though. Just be aware that you may need to apply knotting before priming and even then you might get some knots weeping sap and spoiling the paint finish in the first few summers.
I was planning on doing that, but honestly a good ⅓ of the bottom of the door is fucked.
It was just painted before we bought. Then our tentants smashed in the bottom and blamed it on the oddest break in ever (I assume in reality their son did it playing football or something).
The doors I'm looking at are made from 15mm "good quality European 5th Redwood". Although you've made me a bit nervous about what gets delivered. There's a bit of flex in the price, but as someone else is buying them for us as a gift I don't want to take the piss.