Consider the following as an extension to Rick's comments on POR.
As mentioned, the engine paint is a THICK enamel (NOT the same as regular POR paint). When applying it to cast surfaces, a bristle brush works well right out of the can. However, this will produce unacceptable results on items like valve covers that are smooth. For smooth surfaces, prep like you would for regular aerosol paint application with a quality primer, fine sanding, and tacking prior to top coating. Do NOT use POR solvent to thin the enamel. Use regular mineral spirits from the hardware store. You will have to judge how much thinning to do based on your preferences and equipment but as much as 50% would not be unusual. Then airbrush the paint onto the surface (a cheap single-action air brush from some place like Harbor Freight will be fine). Wait a while for the mineral spirits to flash and then apply a second coat before the first coat completely dries. If you apply too soon, the paint may run. Apply too late when the first coat is too dry and the second coat will lift the first coat. If you are unhappy with the top coat, wait several days and use mild rubbing compound or glazing compound to buff the surface to the level of gloss you want.