Can I use primer as a topcoat?

You can use primer as a topcoat temporarily, but it is not recommended as final finish because primer is not formulated for durability, washability, UV resistance, or appearance like topcoat. Primer's job is to seal, bond, and provide base for topcoat, it is often flat, chalky, porous, and not washable, so if used as topcoat, it will scuff easily, absorb stains, look dull, and wear quickly. For example, drywall primer as final coat on walls will mark easily and not be washable, wood primer alone will not protect wood from moisture. However, in some situations you can use primer as topcoat short term if you have no topcoat available and need to cover, such as using white primer as temporary ceiling paint, or using self-priming paint that is both primer and topcoat, which is designed to be both. For exterior, primer alone will degrade quickly in sun and weather. For car, primer alone will rust and peel without topcoat. So while you can use primer as topcoat in emergency or as temporary coat, it should be topcoated with proper topcoat paint for durability, washability, and appearance. Self-priming paints are exception as they are formulated to serve as both primer and topcoat.