How to remove visible seams between joining objects I have a single object that i have separated into many objects due to sizing issues (level of detail/triangles bla bla) and since separating them there are now seams between each new object in blender. 

here are some examples:




https://gyazo.com/10a8c9008a8419b6499ac17f865e06d9
https://gyazo.com/ed704c4f3d8463b0fb7c1d476940c507

How do I get rid of these. I know it's do-able, however I have not done it in, many a year. 

Cheers
As someone mentioned in the comments, this is caused by how normals are calculated for each mesh you split off. The normals at the vertices along the edge of your split meshes are now pointing in slightly different directions. To fix this, transfer the normals from your original un-split mesh to the parts you split it into. You can do this using the Data Transfer modifier.

Comprehensive steps to fix:


Create a copy of your original un-split mesh and place in the scene. You can hide this mesh.
Split your original mesh into the pieces you want. 
Add the DataTransfer modifier to the split pieces with the following settings


Source object - this should be set to your original un-split mesh copy
See screenshot for other settings