Want to repaint the lanai floor. Do I need to get all the old paint ?

Yes.you have to strip the paint off or the new paint will not adhere correctly

Yes, If it is flaking. Otherwise a sanding first would be good.......

When painting or repainting, it's all about the preparation. Scrape all loose, paint off, sand smooth, use a product like TSP to clean the floor and degloss the old paint. Then paint. Good luck.

No. You do need to prepare the surface well. Feather any hard edges of paint that are missing with a sander. Etch the underlying cement that you've now polished with muriatic acid. Follow the directions on the bottle and wear some protection because if it etches concrete, it will do more than etch you.

Once you've cleaned the surface well, paint away.

Use a power washer. You can rent one with a cleaner bottle attachment, use diluted chlorine bleach, apx 1 cup to 32 oz water. Wear old clothing since bleach will discolor it.

Do not get too close to the area you are cleaning, if you get too close the pressure can damage what you are cleaning.