Filling the inside of an object with some kind of epoxy or resin to reinforce it ... amount of it risks deforming a thin-walled 3D print in the process. For cases where resin would be overkill ...

A finished 3D print ... but can also act as a fill in a pinch. Two other similar methods, one using spray paint and polyurethane and the other using epoxy, are more general-purpose ways of ...