Flush Door are made by using Timber frame which are Covered by plywood or MDF on both sides of door and hollow portion of door filled with cardboards. These Doors are then finished by veneer on top or teak ply etc. These doors have less joints with perfectly flush.
Flush doors have plain shutters so they are easy to maintain and clean as they don't have Designs or grooves in which dust will gather. The doors either have veneer finish or laminate finish.so for cleaning you just need to wipe out door with clean dry cloth.
Flush Door Usually available in market with thickness 25mm, 30 mm and 35 mm. But as per requirement you can manufacture any thickness flush door can manufacture.
Flush doors are generally finished with veneer and polish over it or it may finish with Laminate over ply or MDF used on it.