painting white mdf

Am going to paint some Wickes internal doors this week.The doors have already a white coat of paint on them,I presume this will be undercoat.Do I need to just finish them with gloss using white gloss or should I put another undercoat on them?.Also I have to paint some white mdf skirting do the same principles apply.What are a decent quality brushes the best to buy.
regards

The coating you see is probably factory finish primer..its not as white as you may think, id personally go for two coats of satin or eggshell, for more subtle, IMO white gloss doors look cheap and nasty.

Skirtings...give em a rub down with fine wet an dry paper then a coat of MDF primer or slightly thinned wood primer..oil or water.

Then paint in whatever you fancy...rubbing down between each coat.

Thanks Zampa,satinwood it is then.