Hello guys ,I got a screen and recoat job ,How to judge whether the coat on the engineering floor is polyurethane or aluminum oxide? I know you can do screen on the water or oil base polyurethane ,but you are hardly screen out the aluminum oxide,so I don't want screen out the aluminum oxide,so can you recoat the aluminum oxide coat with polyurethane?

Aluminum oxide can be part of a poly-urethane coating and you won't see it. Whether there is aluminum oxide in the coating or not does not prevent a coat from sticking if the preparation is done correctly.
There are systems for re-coating any poly-urethane coating without any screening.Basic Coating has the Tykote system and Bona has a Bona prep system. Both start out with cleaning any deposits off the floor that may prevent good adhesion. Do you start your re-coat by cleaning the floor? Or do you think that screening will remove dirt and contaminants and allow good adhesion?
Water-based poly-urethanes stick very well to factory coats of poly-urethane.

The choice depends on the purpose of the room. Look at the requirements and instructions.