Somehow missed the post by @Rochdale and will stay with 303 cleaner to be sure there isn't any adverse chemical reaction taking place. Thank you for the advice on that.
I'm thinking of a horsehair brush as recommended by @GOBLUE,
Chemical Guys ACC_S94 Convertible Top Horse Hair Cleaning Brush
and probably 3 coats like @Hi-Step'n370Z did. It's cheap insurance while it's all masked up.
Where do you get the thin plastic sheets?