Making a fully solid STL without any empty gaps inside it

[fred_dot_u]

If the surface of the STL is manifold (has no gaps or discontinuities), you can use Meshmixer to remove the internal flaws. There are two methods. One is to use the Make Solid feature, but if the settings aren't optimized/customized, the surface will change in an undesirable manner. The other method is to use the Select feature, select the entire outside of the model, then invert the selection and delete.

Consider also to post the STL file, as a tiny JPG file is challenging to interpret.

[miozeriux]

Select and Invert feature did it! Thanks champ