If you need to paint sash windows theres a a few things you need to check first.
If there seems to be a lot of rot about the windows then you will need to get a carpenter in to make some repairs. A small amount of rot can be repaired by your painter, or if you feel you can paint sash windows your self then focus on your preparation first and foremost.
Scrape out any loose rotten wood and leave the sash window exposed in the air for a couple of days (hopefully it doesn't rain!), this will dry it out some more for you. After a couple of days has passed you can now apply some wet rot hardener to the exposed rotten areas. The wet rot hardener does exactly what it says on the tin - it makes the wet rod hard. If its hardened you can fill it, if its filled without the wet rot hardener then the filler won't bond with the old rotten wood, the rain will get in again and just make matters worse!
Once all the holes have been filled up you can now start sanding. You don't need to remove all the old paint but you do need to give it a good key. What we mean by a good key is to remove the sheen from the old paint. If you don't key the surface and instead just paint straight over the old gloss then you can look forward to the paint flaking and peeling away within no time at all. When you have all the sash windows rubbed down you should then give them a good dusting followed by wiping down with methylated spirits. The methylated spirits will evaporate away leaving no residue. Once all of this has been completed you can start spot priming all exposed wood and fillers with an aluminum wood primer. The aluminum wood primer is a silver grey primer that will help to protect your woodwork from the elements.
Once the priming has been completed and left to dry, usually over night, you can then start undercoating. For these sash windows in the picture above we used one coat of an oil based undercoat followed by two coats of an oil based eggshell, in this case it was Sandtext exterior eggshell that was used.
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