Flaky paint may indicate that preparation was poor and quality of materials used too. Sometimes best idea is to remove all layers of old paint using heat gun. This is time consuming process and must be done with care. After, you need to prime all bare wood surface (doesn't matter - water or oil based primer). Fill holes and damages using two parts wood filler, usually 1 undercoat is enough and then good quality exterior gloss. You can also give a try to Dulux Weathershield System - guaranteed for 6 years, but you must use 3 Weathershield products (primer, undercoat and gloss).

Hi
Iv been a decorator for 30years now
Just give them a scrape down then rub them down with 40grit sandpaper it must be 40grit or your wasting time, then just coat up bare wood with oil based wood primer, then undercoat & gloss them.

As all other answers, prep work is most important. Scrape all loose paint off and thoroughly sand, avoid the use of a heat gun you could crack the glass if your not careful!
With rot, remove as much as possible and use ronseal wetrot treatment to harden the wood that remains. Fill with a 2 part fller and sand down smooth.
Weathersheild paint is the best for this, it used to come in a 3 part system, 1 preserver, 2 undercoat, 3 gloss, the presever is no longer available due to voc's
I use a good qaulity wood preserver (not ordinary wood primer) I have seen plenty of windows fail with oil based wood primer from as little as 2-3 years!
2 undercoats, cut in to the glass by a mm leaving no holes for water to get in under the paint.
A nice light sand and then a full coat of gloss

hi im glyn i have been a decorator for 26 years, scrape off any loose paint or if they are very bad you may have to burn them off, prime any bare timber with an oil based primer then give them 2 undercoats and 1 gloss. also make sure you rub down between coats and use a good quality external paint (dulux weathershield)

you have a lot of conflicting answers and everybody has there own way of doing things you can scrape down sand prime apply two undercoats and then gloss but if you are a real perfectionist then i would burn off all the paint first with a hot air gun making sure you have with you a water pump for safety sand down and then use to coats of flexible undercoat which can go on to bear wood and then the flexible gloss will last for years the system is from layland paints hope this helps colour/sense decorators

Hi this David from colourways ive been a decorator for 35 years i always do a good prep before i paint, one oil based primer, two undercoats,one gloss,

COLOURWAYS

if it needs filling use 2 pack wood filler. check the window putty for damage. if it is pick it out and replace. always go for a good quality paint ie dulux trade. but if you want a eggshell finnish make sure it a exterior one and i personaly perfer to give 2 coats of gloss or eggshell, extra protection against weather and u.v ... roy

Do NOT burn off at all as this is a major fire risk to your home. If you get hold of some 40 grit oxide paper then wear gloves or expect to rip your skin off ! If you have old windows or new a 'scarcen scraper' with a tungsten-carbide blade from a good retail decorating store or dare i say B & Q costing £10 to £12, this will take off all the loose paint, then use the dulux paint system. Good luck

remove all loose, peeling or otherwise defective paintworks from existing windows by: thoroughly rubbing down and raking out, and then fill and paint with oil based microporous undercoat paint 2 coats then 2 top coats with oil based finishing paint. once painted fiil around frame with a external frame sealant.