Zinsser bin and undercoat

No, zinsser is brilliant will stick to anything! Good undercoat/stainblocker. Very watery a little goes a long way.

It will tell you on the tin that it sticks to glossy surfaces but just to make extra sure, especially as its a bathroom and prone to condensation, I would give the walls a light sand just to get rid of some of the sheen. Then a coat of your 123 followed by a couple of coats of your finish coat. Incidentally Zinsser BIN is a different product to Bullseye 123. BIN would not be as suitable for the job you are doing.

Hi, it is all wats good to sand soft seen walls to provide a key for the next zinsser to grip on to. Once the zinsser has dried apply emulsion normally.

If the walls are in good condition, you may not need to sand them before applying Zinsser 123 Plus. However, if there are any glossy areas or imperfections on the walls, sanding them lightly can help to create a better surface for the paint to adhere to. Zinsser 123 Plus is a primer, so it should not require an undercoat. However, if you want to achieve better coverage or hide any existing colours or stains, you could apply a coat of watered-down emulsion before applying the topcoat over the Zinsser. This can also help to create a smoother finish. Once the Zinsser 123 Plus has been applied, wait for it to dry completely before applying the topcoat. Make sure to stir the paint thoroughly before use and apply it in thin, even coats using a roller or brush. It is recommended to apply two coats of the topcoat for best results. Before painting, make sure to protect any areas you do not want to paint with masking tape or plastic sheeting. Also, make sure to use appropriate ventilation and personal protective equipment when using paint and primers.

Depends of the conditions of the walls.

I would do the following process -sand walls preferably with a dustless sander to remove the sheen, undulations and imperfections. - clean walls free of dust -prime with your product of choice I'm assuming you are using BIN for a particular purpose. -fill any minor imperfections, sand entire surface with 320, clean free of dust and second prime. (if there is large filled areas I tend to apply some gardz or peel stop as an extra support before 2nd priming) -give the whole room a light key with 320, clean free of dust and top coat with a quality bathroom paint: luja7 is my go to by tikkurila.