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I’ve got Velux loft windows with a varnished pine wood finish. The Velux instructions recommend stripping and re-varnishing them every 2-4 years:
However, they look perfectly good, there’s no cracking of the finish or anything. Is there any logic in doing this? I’ve never heard of regularly stripping and re-finishing wood except exterior wood (the wood on these is on the interior, not exterior).
I think you're right to be a little sceptical of this suggested refinishing schedule, even with Velux obviously being the expert on how their products hold up in real-world use.
In general there's absolutely no reason to completely remove finish that is still in good condition. It is appropriate to remove and replace finish that has degraded, and it's something that should be done more on furniture that people care about that they want to keep in good condition1, but excepting pretty excessive amounts of water exposure and/or UV damage (and most of Blighty just doesn't get that much sun!) I would expect the finish Velux use to hold up better than a couple/few of years, as it appears it has in your case.
TBH I'd expect at least a 4-5 year service life and 10 certainly seems possible — I've seen older Veluxes, in situ for far longer than just 10 years, that I'm certain had not been refinished but had no obvious degradation of the finish anywhere, in other cases just obvious edge wear from being scraped by the opening rod with the rest looking fine..... from the floor at least.
So have a good look at the finish up close so you can see it properly, and if it is in good shape you should be fine doing nothing for the time being.
Another option
Instead of doing nothing or refinishing completely there is a middle-ground option, topping up the finish. This can be done as a means of extending its useful life, and done with care and discretion you could essentially stave off a complete strip and refinish nearly indefinitely (possibly beyond the service life of the window).
The way to do this is just like how exterior varnishing is often refreshed, some sanding followed by application of fresh varnish. In exterior work the sanding would be more extensive and heavier, to remove degraded finish from the surface and reveal cohesive varnish beneath, in this case you'd just be sanding or scuffing very lightly to provide a 'key' to aid adhesion of the new varnish.
It's wise to also clean the surface in addition to light scuffing/sanding, especially if the finish might have been exposed to some oil (vital in a kitchen obviously). Warm soapy water is OK for this, sugar soap solution could also be used and will be more effective, but in either case the surface should be very thoroughly rinsed off with clean water afterwards.
For this kind of top up I would recommend wiping varnish, which doesn't have to be bought as it's just normal oil-based varnish diluted with additional white spirit (US: mineral spirits). Decant a little of your varnish to a clean container, then thin to wiping consistency by diluting approximately2 1:1. You're aiming for a pretty thin consistency that will wipe on thinly and easily, and also allow the excess to be easily wiped away after a couple of minutes.
1 Including on antiques, but that's an argument for another Question.
2 Be guided by the original varnish viscosity and the viscosity you want, don't follow online formulas by rote as all varnishes vary to begin with. And additionally an opened tin of varnish will invariably thicken up slightly during storage so the same varnish will require different amounts of spirits the first time and the fourth time the tin is opened to provide the same working consistency.
My advice is you revarnish only when you find it necessary. Why? Because many manufacturers give instructions to get themselves covered. The loft windows will always be in good condition for over 5 years unless they are exposed to very harsh conditions. The last time I revarnished the Velux loft windows in my pool table room was 2 years ago and that was after using them for 8 years.
