Zinsser Bin sealant, can you educate me?

Water based paint is very unlikely to cover this straightforwardly I think - as I believe this is shellac. I covered some MDF with a similar product and had to sandpaper the surface to provide some key.

It still took lots of coats (4?) to cover nicely and as I was desperate to hurry up you can still see where the sanding was less than perfect if you look carefully (and know where to look).

It took a long time to dry between coats. I think you can do it if you persist.

I have decided I hate MDF even more now! I wish I hadn't let my carpenter use it to box in my bathroom pipes.

Oh, sorry, I see you have put an oil base over the zinnser...
I think you will have to use another oil on it I'm afraid. You could try keying into the surface with sandpaper but I'm not optimistic.
Sorry. I think I misunderstood.

I'd stick to either oil-based or water-based for all the coats on a surface. It's important to sand with fine abrasive paper to provide a 'key' (tiny scratches/roughness) for the next layer to bond to.

I've found that only oil-based will do for windowsills - as I have a cat!

If it’s MDF I think you need to use a water based sealant and primer. I loathe water based acrylic style household paint. The finish is never as good and hard wearing as oil baised paints as they don’t ‘flow’ very well. You can buy flow medium that’s supposed to help and you are supposed to use an artificial hair brush for the best finish. Apparently oil based paints are being discontinued and acrylics will be used instead.

Environmentally speaking I don’t think plastic paints will be better for the environment as we all now know about micro plastics. Maybe best to return to wood and beeswax for the most environmentally friendly results.

The problem may be the water based paint over the oil based primer.

I'm so tired that I thought the heading was about a Middle East leader hahahaha.

Ellenvannin, oh that’s funny. , it does sound like a Middle East leader. ?

EV so did I!