I'm having a window and some French doors made for an extension. They are being made in Redwood and I'm thinking of using linseed oil paint to finish them which I'm hoping will prolong the life by avoiding getting any moisture trapped in the timber.

The joiner who is making them usually treats softwood for use in external applications with Cuprinol clear wood preserver prior to painting. I'm wondering whether this will interfere in any way with the linseed oil based paint? I will be treating with linseed oil prior to application of the paint in any case. Should I let him treat the frames with the preserver or not?

If the 'redwood' is larch, Douglas fir or a decent Scott's Pine then don't bother with the Cuprinol. Linseed oil followed by four thin coats of real linseed oil paint will do just fine. I use the Swedish made Alback, supplied by Holkham Paints.

The cheapest wood that they can buy in straight pieces

Treatment is not necessary unless you are thinking that the window wood will be getting wet and here lies the problem. They do often gets wet by design! because of insufficient falls on flat areas, water traps especially where the glass sits, water designed to run into the joints especially in the sills, square dather than sloping down and out glazing rebates, no water stop or drip etc.

I always drill holes in the members under the glass to let water out in the unlikely yet pretty well certain event of it getting in. I do not paint the underside of things behind the drip. This is mega important as it allows the joinery to dry out in the event of it picking up moisture.

Microporous paints are just as porous to water as to water vapour so were a conn -- go with linseed oil paints.

Bl**dy craftsmen, always taking holidays! But hey, Chris, take a hliday yourself and go wih the flow :)

"redwood U/S" means redwood unsorted, which weirdly means the best grade of redwood, certainly not the cheapest Biff! We used to find machine gun bullets in Russian redwood! No joke when putting it through the circ saw!

Cuprinol or a straw coloured basecoat will not have any detriment on your linseed finish. It soaks into the cells of the wood.

Not sure how good it is in principle for the enviroment.

I use Kingfisher chemicals straw basecoat, google it.

I have had 30 years experience of treating exterior joinery for my own use. I would NEVER fit any external joinery, whether hardwood or softwood, without treatment with clear cuprinol or equivalent.

Most paints and oils try to act as water barriers and fail - they merely obstruct drying out of wet timber. Presevatives kill the bugs that cause rot. Merely 'painting' on preservative is virtually useless. Water (the enemy which supports rot) travels up the grain easily, so the same route is necessary for preservative. All timber is naturally resistant to penetration across the grain. Stand a stick of dry timber in a bucket of preserrvative overnight, and be amazed how far up the presrevative is sucked, and right through to the core.
The joinery must be constructed of dry seasoned timber using cascamite glue and immersed for 7 days in cuprinol - maybe one edge at a time if a window frame. (check other glues for solvent resistance). Drain off and leave to dry in a ventilated space for 4 weeks, after which you can paint etc as normal for decoration ( I have had no adverse reactions), but the preservative alone has a very long life without paint. Omitting paint can be a green plus as well as a labour and cost saving. It also means that 15 years later you can 'paint' vulnerable parts with a refresher dose of preservative in very dry weather for added surface layer protection.
Not all timbers absorb preservative well: generally those most naturally durable tend to absorb least and vice versa - see info on the web.
At the very least stand joinery overnight in preservative so that vulnerable parts have end grain immersed.

If you need to construct a trough for immersion, a lead-lined trough made from planks is easily made.

Linseed oil is the perfect thing to use on the oak doors -- please dont use anything else.

You could try making new windows from oak! Stain and wood dye are the same thing to me.

I always use linseed oil on oak.

It depends on the linseed oil - it's not all the same. I find the Swedish cold pressed raw linseed that Holkham Paints supply is not only more expensive than most but it soaks in quickly leaving a completely dry surface next day that is not in the least bit sticky. I've heard tell, but haven't tried it, that the linseed oil sold as a feed additive for horses has similar properties at a lower cost.

Matching two different woods to the same colour is probably a fruitless exercise as, even if you achieve a match, it won't stay like that. The two woods will change colour, probably at different rates. Best just take delight in the variety.

The doors will need no more than the oil a few coats several weeks apart to start with then once a year possibly twice on the bottom parts.

The windows I cant tell you about, but I will say that the best successes I've seen with these have been without any protection what so ever 50 yrs and still no rot compared to the rest of the houses around with yucky so called replacement windows!

I think I'll apply just linseed oil to start with and see how they look (can always paint the window at a later date). They will be located on a south facing gable wall (single storey) but it has an overhang of almost a metre so that will give some shade and keep the rain off to some extent.

I was looking at the Holkam website and it suggest using "shellac knotting solution" on the knots before painting but also on the rebates and the beads before glazing. Why is this? Is it necessary (because I'll need to order some)? Can anything else easier to obtain be used?

There was sone comment about the linseed oil having an adverse effect on silicon in the glazing units??? What diameter holes do you drill Tony and at what spacing?

I think the patio doors I've got are factory glazed so I've just got one window to fit and glaze with sealed units. Should I order some shellac and paint the beads and rebates then put linseed putty behind the beads or should I just use silicon? Would the silcon, if used, be adversely affected by the linseed oil?

Chris, I do one 10mm dia hole 65mm from each side. On fixed glazing I angle the hole so it comes out under the cill but outside the wall.