Let me first say I've never been to Finland or to anywhere with temperatures such as you are often exposed to...So I have a question - but it is only a question!
How old is the rotten wood? I ask this because as I am sure you already know; there are a lot of other factors beyond simply being "wood" that need to be considered in answering your question.
Will the finished product be painted?...or will it be stained?
Is it a part of the window that houses glass?
Is/are the part/parts going to be on the interior or the exterior, or is there some of both?
Here's why I suggest having all of these answers. If the part is to be painted, you have much more freedom with your material choices than if the wood will be stained and needs to match color, shade and grain pattern within reason.
If the part houses glass, there may need to be special allowances made (dimensionally) in order that expansion and contraction (due to seasonal changes) doesn't pose additional stresses.
How old is the wood being replaced? Let's say window wood lasts 5 years in your environment - I would say you need a more durable material than that type of wood. If it took 20 - 30 or more years (the word "historic" makes this come-to-mind) for the rot to occur, I would think that a structurally-sound epoxy repair that provides owner-approved appearance MIGHT be a good choice.
It seems to me that this may be a great time to sit-down with your customer and explain what you can and cannot do and the pricing for each method. 35 years ago, I learned a lesson in a very expensive way - and my personal expense was HUGE. I will try to parallel this to what I believe your current situation to also now be.
My situation 35 years ago: I owned a small construction company. A conscientious contractor had built 28 houses along both sides of a single street. A material salesman convinced him that if "his brand" of waterproofing was utilized, there would be no need for an exterior drain pipe. I was in no way involved in that new construction process.
Within a year, I got a call to come-out and place a bid to repair all 28 leaking basements. I gave a fair price for the work, the builder assured me he was going to pay for all of the work, but he said my price was more than he could afford - so I left. I never expected him to call me back. A couple of weeks later, he called me back and wanted me to meet with him on that street at one of the houses. THE SAME material salesman was there! Since I had nothing to do with the new construction a year earlier, I was not (then) aware it was the same idiot. That guy went into some long story about "his solution to the problem". I refused to do the work - I felt it would not work and I even pointed-out that doing it used two materials in VIOLATION OF WARRANTY REQUIREMENTS! Again, I left hoping to never hear back from either of them!
Another few weeks went by and my phone again rings with more calls and the salesman begged me to do the work. I told him that I could not do the work because I had no expectation of the work to perform as he insisted. He gave me every kind of assurrance, he even had his boss come-out and insist it would work and he had the owner come-out and hear everyone tell how great this would be.
HERE'S WHERE I SCREWED-UP: I should not have trusted the lying, good-for-nothing material salesman - because this "got the monkey off his back" and "put the monkey on my back". I was raised to be honest and keep my word, my handshake is as good as a written contract - BUT THERE ARE MANY PEOPLE THAT DO NOT WORK THIS WAY! I worked on 28 houses and was paid promptly by the builder for very low bid amounts. The big problem is that I had been talked into personal guarantees of all of the 28- basements being dry in order to cash the checks! Well, everyone of those houses (LATER) got a correct waterproofing repair - AT MY EXPENSE! Joy is an angel for not leaving me during this terrible financial time!
Now, Tiny my friend, here's how I see your side of this "playing out"...By your answer to Dan's question [above] it makes me guess that you haven't used this method in this application in this scenario before now. If I am wrong, I sincerely apologize! Let's say you call the guy that sells the epoxy for the window repair. I know nothing about this person - okay? But he is quite likely expected to PROMOTE THE PRODUCT. He WANTS TO SELL YOU THE PRODUCT! Remember, I cannot speak for the person you will be working with - but there is a chance that you may be told something that is either not true or highly unlikely.
Here's the repurcussion. Let's say this ends-up being a faster and easier way for you to get your work done. You may then, bid the project for less money - TRYING TO BE HELPFUL TO THE OWNER (exactly what I did). What will happen if the product fails? Who will be there to stand behind it? If you're the one that did the work - MOST LIKELY YOU WILL ALSO GET THE BLAME.
Tiny - I hope this information gives you some good food for thought,
Otis Guillebeau from Auburn, Georgia
OPG3 Tweak everything!