I would first need to see a jpg of this to get a better picture, but I would start with Join select 1 surface then mb3 inside the join box select distance progation. check the two boxes tangency and conn boxes to show gaps. Then see what comes up in RED and fix those surfaces. extract the part and remove the bad sketches or items that were created or imported with poor practices and you will end up with a dead solid that is closed. Then I would recreate the data with best practice methods so it can be modified easily...
As a Tip the boundry pic works to help find issues also...
Also some people make the big mistake of projecting live in sketcher which always errors. I would recreate those lines if possible or isolate them.
