Bug Bear et all - I have this circular argument on almost every job I spec Linseed paint on, then have the circular argument once more with the decorating contractor.....
The problem is "normal" paint is rubbish so its just not worth using. Not if you want a long lasting quality job in any event.
Most stuff painted with modern paints (especially in high exposure areas and on things with moving parts like windows) starts to fail after 3-5 years and you have to repeat the process. Expensive and a waste of time and puts historic joinery at risk of decay.
That is because very broadly paints yeas ago contained heavy crude solvents or lead - which although was not good for your health actually made some good quality paints that flexed with the wood and lasted. You cannot use these paints anymore (except lead paint if is a listed building or for boat building but I never use this) so you are left with the rubbish available now.
Georgian and Victorian paints were usually Lead based but with Linseed oil in them also. The modern Linseed paints obviously do not have lead in but are very similar in performance.
Modern paints such as Delux Weathershield for example basically have solvents in them that are EU compliant and in short makes the finish go rock solid - ;like a sheet of plastic over the wood which presents 3 problems; (see data sheet)http://media.builderdepot.co.uk/media/c ... 470806.pdf
1) It does not move so when the wood expands and cracks the paint does too.
2) Once cracked - which it will crack - it lets water in, but it cant get out so it rots the wood.
3) Once cracked it splits and breaks off - e.g it does not soak into the grain.
Microporous paint is a joke really and does not provide the breathability required for timber. Especially softwood which moves more than hardwood.
Also, why are we using these chemicals on windows - here is a list of the solvent make up of Dulux Weathershield ;
Contains; 3-IODO-2-PROPYNYL-N-BUTYL CARBAMATE, ETHYL METHYL KETOXIME
Its bad for the environment and just unnecessary as the product does not perform well either.
Linseed paint soaks into the grain and is very flexible and breathable, meaning that it sticks to the wood and stays there. Its more like a stain in performance than a paint. So, 10 years down the line the window may look dirty and need re-painting but the wood will be basically still covered and protected.
Also we need to think about why wood rots ? You need a sustained moisture level over a long period of time, so in theory if windows left unpainted - if they can dry out they will not rot. So we can see why Dulux and the like rots the windows as it holds the moisture in, thus meaning sustained elevated moisture levels + Rot.
As for application, Linseed paint is just paint, its just slower to dry than modern paints with massive amounts of chemicals to accelerate drying. Its no slower to dry than lead paint. You can put it on with 3 coats with little else other than a paint brush. Its really really simple.
Coley is clearly a craftsman, demonstrated by his fantastic work posted on here and is looking to obtain a absolutely first class finish ,so is going to great lengths to obtain the best finish possible and experiment with Linseed paint - hats off to him he's a craftsman. However, you don't need to do this to get a decent window and the reality is, there is no actual difference in application to "normal" paint - example
Dulux Weathershield has 3 parts to their "system" - a liquid you brush on the wood, an undercoat and 2 to coat - so 3 tins of paint.
Linseed paint - 1 tin of paint and some linseed oil to thin down the first coat if you want.
Both systems use a thinner - white spirit / balsam turpentine - basically the same thing.
Both systems use a paint brush
Coley - I would suggest you phone up Michalus from Oriculcum who runs the firm as he is a very helpful chap and will talk to you about issues you are having. He has decorated some serious buildings and has many many case histories and examples of the paint being used to great success and lasting a long long time.
I have always bought from them as they are the only people I am aware of who have been operating on a consistent basis (e.g not running out of paint etc), they are also the cheapest all things considered, and the chap who runs it is very helpful and will always discuss projects with you.