How to paint glass or plastic white so that is fully opaque?

it just depends on what the door is made of. I would try contact paper rather than paint since the paint might not stick very well,

Hi Kiki- because doggie doors are meant to bend I think perhaps a type of glue that you could paint on might be a better alternative. I think regular paint would just flake off.

Rustoleum 2X or Krylon spray paint for plastic.

What is the door made of? If it's plastic or metal, I would use an appropriate spray paint in a dark color.

pet doors have a plastic or acrylic flap I have never seen one with glass- be sure the paint you use is pet safe so that it won't harm your pets

https://www.petdoors.com/blogs/dog/pet-door-decor

you could also buy contact paper that comes in a opaque design

https://www.amazon.com/privacy-contact-paper-windows/s?k=privacy+contact+paper+for+windows

The animals going thru it may take the paint off. Consider contact paper. If it gets worn, cut another piece. Also consider if the animal(s) will push open a solid dark door. See if it will bother them by taping a very thin piece of cardboard on it first.

My cat loves his clear plastic door, its his window to the yard to see the weather, birds, deer etc. And if he wants in he peers through the window, and the feral cat sits right in front of it to let me know she's there to be fed.

But if you want to paint it, buy Krylone or Rustoleum plastic spray paint.

Generally the top portion of the dog door is glass. Are you going to cover that as well?

Your pet may get confused if the door is painted

I hope this helps:

https://www.craftsbycourtney.com/how-to-crafts/glass-painting-with-new-decoart-opaque-glass-paint/

Opaque glass paint does exist!

https://glasspaint.com/opaque-glass-paint-does-exist/

You could spray paint it or cover with contact paper

Hi! There is contact paper that looks like an opaque window film. I would use that instead of paint. Good luck!

I don't think you need a specific type of paint. I would recommend a paint with a more glossy finish if pets are walking through. A glossy finish will make it easier to clean.

Any local hardware store or paint store will have spray paint meant for plastic...

You could spray paint

Here's a tutorial that might help - https://chascrazycreations.com/painting-glass-diy-guide-and-tips/