Should I use mist or prime on old pink plaster before top coat

Use a product called zinsser gardz first to seal in any left over wallpaper paste the carry on as normal

Allow the wall to dry, then place some quick drying filler onto a scraper and work it into the crack or hole, making sure you fill it completely. Scrape off any excess filler as you go. Follow manufacturer’s instructions on drying time before continuing. Use fine grade sandpaper to sand down the filler until you have a smooth and level surface. Cover the floor with a plastic sheet to protect the floor from paint splatter. Using a 2–3-inch brush, start by applying the primer to the edges and corners before starting on other areas. Pour the primer on a paint tray, then roll the roller on the primer. Paint in a V or W shape instead of a vertical strip – it helps prevent uneven coats of paint. 2-3 coats of primer will ensure good coverage between the wall and the subsequent painting.

A mist coat should do it

I always apply a simple mist coat and it's never gone wrong. Get some Trade Matt emulsion, (Leyland is cheap and good enough for a mist coat, very easy to find in stock) and water it down a bit. New plaster can take an extra coat as it can absorb a lot more.

Hello , you should I add mist coat Thank you

Yes will defiantly need a mist coat