i have bought some unused but old timber window frames cheaply. they are made by premdor and supposedly for 20mmm units. they are i think basecoat stained but this is worn and scratched in places. I want to use them and understand from discussions here that linseed oil paint is the best thing to use, can this be used without removing the basecoat, would i need to sand them ? I also understand tha a 20mm unit isn't as good as a 24mm unit. could i fit a 24mm unit anyway perhaps cuting down the wooden beads that are suplied if nescerry ? is it worth it? I gather that krypton gas would improve the proformance of a 20mm unit how much more expensive is it and is it worth it?
i'm on a very tight budget so have no chance of affrording new scandinavian tripleglazed windows or anything but still want to do the the best i can. so your advice would be welcome.
by the way the building i am doing counts as a mobile home and therefore does not need to comply with building regulations.

argon gas fill is about £6m2 +vat
and gives a potential decreased U value of between 0.1 - 0.2 W/m2K

draught proofing them well , would be better and quite cheap to achieve

How wide is the existing beading? If there's plenty of extra width, it probably wouldn't hurt to cut it down to allow you to use thicker units. Alternatively, would the section of the frames allow routing out a little extra rebate depth? Thicker units are more or less free extra insulation, so it would be worth a bit of work to fit them.

Don't we agree that siliconing-in, as is most often done, is the main 'cause' of DG unit failure? A DG unit is much happier for longer in a dry-glazed drained situation. As rogerwhit says, use two 1mm or 1.5mm dry glazing pressure tapes, no mastic, and work on the assumption that a little water and/or air pressure may sometimes enter the rebate space, so make sure it can drain and ventilate out the bottom - either by drillings up through the bottom rail or ideally via an aluminium e.g. Exitex bottom bead. This all adds even more to the required rebate depth. You can make do with slimmed-down glazing beads, or as suggested above, machine out a bit more rebate depth. The inner 'fence' could probably go down to 10mm.

I strongly second what fostertom says above about dry glazing versus silicon for DG units. Dry glazing tape and a carefully arranged vented rebate is very much best practice, especially with timber frames. To facilitate venting, you want the unit to be a little bit smaller all round than a tight fit - depending on the rebate and beading, maybe order them a minimum of 5mm smaller than the tight fit size in both dimensions, ie. 2.5mm gap all around, with the unit stood on 2.5mm spacers. A bigger gap is better if circumstances allow, but the glazing tape needs to be seated on glass and not onto the edging tape of the DG unit.