do you need to use water based primer and undercoat if you are using a water based gloss

While it's not that important and the paint will stick, the paint manufacturers formulate their products accordingly so I would always use the same brand or at the very least, stick to a water based primer undercoat although if oil based undercoat has already been applied I would not re undercoat, I would just apply 2 coats of the top coat.

No , it does not matter if your primer or undercoat is water based or oil based if you are useing water based gloss. When useing this type of gloss paint use as a undercoat. 1 coat primer . 2/3 coats of water based gloss.

Short answer is yes. I always use a good quality trade paint not own brand from the DIY sheds and I find a combined primer/undercoat works best. I make a lot of stuff from MDF and do not find it is necessary to use a specific MDF primer Hope that helps Gavin

I agree with Proctor decorating. what you should be aware of, is water based gloss is not as hard wearing as oil based and outside I would definitely use an oil based gloss and maybe 2 coats of that.