If you function in a moderate industrial or marine environment, here is the ideal single pack epoxy primer with next-gen pigments to protect your steel structures. You get excellent adhesion and corrosion resistance in adverse climatic conditions.
If the previous coating is in good condition and requires no levelling, then putty is not required for repainting.
However, in cases where the previous coating has deteriorated leaving behind uneven surface, or in order to fill nail holes and small cracks, putty will be required.
Primers are designed to improve adhesion and bind the surface. It cannot withstand the attack of harsh elements. If you don’t apply a coat of paint over the wall primer, it will most likely deteriorate and turn into a chalk-like powder.
The role of the putty is to smoothen / level; the surface and it is essentially an undercoat. It should under all circumstances be over coated using a primer, followed by two-three coats of emulsion.
Ideally puttied surfaces must be overcoated within 5-7 days and not left exposed.
It is not recommended to add colour to putty.