Smooth the surface
A surface is only as good as its substrate. This is the case for all surfaces and particularly the case for surfaces that have specific functional requirements or will be subjected to demanding use. Correct preparation of the substrate is thus an absolute must prior to the application of any form of coating, irrespective of its nature.
An even, very smooth surface is indispensable. So any cracks need to be carefully filled while porous or highly absorbent substrates need to be covered with a suitable primer. If there is the risk that fats, oils or dried water stains could show through, the corresponding site must be additionally sealed.