Exterior painting of window frames.

Yes, very much so. Remove all loose existing putty and then replace with new. When you want to start doing it take out a ping pong ball sized amount and knead it in your hands for ten minutes to make it pliable, then squash it as tightly into the edges of the glass and frame. Put more in than you need to match up to what is there already! Then using a putty knife, even a cheap one will do if you are not going to be doing much of this work, lean the blade of the putty knife against what is there already and then, with a lot of force to maintain the position of the knife against the new putty push and cut it into place. Let it go off according to the putty makers instructions and then you can paint it. Sorry this is a tad American, but it's a good video showing you the basics... https://www.youtube.com/watch?v=3O6ZsKdw8IE

After you you have replaced the putty leave to dry for a day . Then prep the windows and use a wood primer for the first coat (that's a must ) then your undercoat then gloss