I am a decorator, I'm not new to this, I'm getting on a bit, limewash and distememper were still in regular use when I was an apprentice. I specialise in conservation and restoration of older properties. I carryout my own joinery repairs, Any joinery repairs are treated with this.
Valtti Primer, priming oil.
Very expensive, but does the job.
It prevents the uptake of moisture.
I use it prior to finishing with whatever finish the client chooses. Stain or paint.
Sadolin Classic should be applied after the wood has been treated with a good quality wood preserver (if Valtti is cost prohibitive). I would not use Sadolin Classic or Extra or indeed any other woodstain on untreated wood, they are simply not thin enough to penetrate deep enough, and are not as effective as clear wood preserver at killing and preventing further infestation.
FWIW, I would never use a clear varnish on any external wood.
Those black spots are likely to be signs of fungal infestation. Black spot.
Strip the existing coating, wash down and apply Fungicidal solution, paying particular attention to end grains and joints, allow to dry, or apply the Valtti Oil Primer as per the instructions.
If using a water based finish leave the Valtti for at least 5 days!!! if using an oil based product 24hrs will be OK.
