Imperfections on surface with oil based paint

Clean the architrave with a soapy solution if is dirty or greasy. Sand it - all of it. clean all the dust with a cloth or very well with a dust brush, prime it, leave it to dry then fill all the imperfections with wood filler. Can be 2 parts of wood filler or ready to use filler. It is not a high traffic area for a very strong 2 parts filler but is your choice at the end. The difference is that the 2 parts filler is very hard to sand. Fill a wider area than the actual imperfection as when you sand it you need to sand it smooth edges till you feel no filling edge when you go with your hand on top. Use 120 grit sandpaper. When you are happy with the smoothness, prime all the filled area again and leave it to dry. For joining gaps, use a flexible caulk and leave it to dry completely b4 painting. Give it a smooth 240 grit sanding everywhere, dust it and apply the topcoat. Protect the environment using water-based products. :)

You can use ronseal high performance two pack filler, this is basically car body filler in white, so excellent adhesion and can be sanded to a perfectly smooth finish. Once all prep is done ( sanded etc) yes use a primer/undercoat. I cant recommend what because you haven't stated what paint system you plan on using

light sand the areas that need filling and fill as normal sand touch up with oil base then full coat..to finish coat, you will be fine.