Painting.

Painting is not my favourite pastime.

I use paint of two different types. For any work which will need rubbing down and is effectively being used as a filler I will use a spray can. Normally a primer, it is easy to apply, quick drying and easily rubbed down.
If the body is good, and I am priming to take a top coat I use either a Humbrol of Railmatch primer with an airbrush.

I would guess, (if the problem has not already been resolved) that several thin coats of primer after filling the crack may do the job. And i mean, thin coats...

Peter