Gloss on woodwork turning yellow??

[Maty]

Less than 12 months ago we glossed all the wood work in the house (doors, door frames and skirting boards), a job I quite simply hated! In most places this has now turned a lovely shade of yellow that I would expect to see in a house occupied by someone smoking 200 fags a day. It therefore needs doing again, but what is the correct process? Last time we just gave everywhere a quick once over with some sand paper then set to it with the gloss, I understand some form of undercoat should have been used???

Or, are there any alternatives to using gloss that might stay white for longer, again what method should we use?

I don't mind doing the job again but it needs doing properly this time.


Thanks

[Maty]

Do you have to undercoat first then, or just sand and paint??