Surface preparation is a must before painting because the total life of a paint coat depends to a great extent on how carefully the surface preparation is done.
   

1 Surface should be thoroughly cleaned to remove all loose particles.
2 Surface should be degreased by suitable cleaning solution
3
Rusting on the metal, if any, should be removed with the use of suitable derusting process. (eg. hand tools, sanding, acid pickling treatment etc.)
4 Repair any cracks, holes or other imperfections with suitable fillers.
5 After the filler dries, sand to smooth finish, with a damp clean rag, wipe away all the loose dirt and dust..

 
Note:. The surface should be perfectly dry prior to painting.Use appropriate primer, putty and finish paint as per the manufactures instruction .

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