I think you are misunderstanding the purpose of Gardz. It is not meant to 'fill' anything. In fact, it is a very loose bodied product...almost has the consistency of water. What the Gardz does is keep the paste from screwing up your latex topcoats. If you painted over paste, the paste will eventually cause the paint to crackle away. The Gardz will keep that paste locked down so it cannot affect your topcoats.

In your case, the best solution is sanding followed by skimcoat where necessary. Steaming will do nothing because the paste is already painted over I would use an orbital sander and yes it will be dusty and messy but it's fast. You could hand sand but this method will take forever. You could also do no sanding and the skimcoat over the rough surface but I think buzzing it down with an orbital is your best route.