So, a little background. I have long term borrowed a friends XYZ Da Vinci Jnr printer. I have had good success with printing models from Thingyverse and the like. I recently wanted to try a bigger project and wanted to print out a prop gun from an online computer game, Destiny. There's a site called destinystlgenerator that rips the models from the game world and allows you to use them. They're a little complicated as they contain about 40 different files of various parts of the gun. I have used MeshMixer to strip out the unnecessary stuff and save as an STL file. However when I open up the file in XYZ software it seems to be made up of the individual triangles, not a solid piece. I wanted to check it was looking right in the print software before I tried to split the file, as I want this thing to be life size, which would be nearly a meter long!

Pictures attached. This happens if I merge all the parts together or not. It also happens if I open the saved stl in MeshLabs, edit a little to close holes or something, and then re-save.

Any help in getting this model clean enough to print, or to print from a different software (without totally hacking the printer, as it's not mine) would be appreciated. I also need to make this model solid at some point which is proving tricky, any extra advice would be much appreciated.

Cheers