From the instructions on the Kilz™ website for Kilz Original:
Wash off any dirt, grease or smoke damage with a non-soapy detergent or a TSP substitute, rinse well and then allow to dry.
From the instructions for Kilz Odorless:
Wash off any dirt, grease or smoke damage with a non-soapy detergent or a TSP substitute, rinse well and then allow to dry.
Given that the success of any
paint job lies in the preparation, and the fact that Kilz recommends the surface be cleaned, I would clean it well.
If you are using something other than Kilz, I would check the instructions on the can (which can usually be found on the company's website also).
Raya