What type of primer / undercoat do I need?

Jet

Previously glossed (oil based) skirting boards and dado rail.

I want to paint them in Satinwood (water based).  I have sanded down the old gloss but am I correct in thinking I need a primer / undercoat now before the satinwood paint?

Thanks.

Grenage

You only need primer on bare wood.  Whether you need an undercoat is down to the paint.  I usually use Johnstone's Aqua, which requires an undercoat.

Albermarle

When I last did this I just dusted and wiped them and then painted them directly with the water based satinwood.
The first coat went on OK in some places but was a bit streaky/patchy in others. However the second coat went on fine.
Probably a bit of a lazy way of doing it though.
I noticed when I changed to a trade satinwood, from a normal one, it went on better. ( Although being an MSE person it was Wickes Trade)