I'm trying to combine two different meshes, and in these three small parts one mesh has more vertices than the other. This makes it so I can't just pull the vertices together and remove doubles, and it creates these gaps in the mesh. How do I fix this? Can I add more vertices to the other mesh?
Do not create new vertices, just snap the existing ones to each other. Activate snapping (Shift S), choose Vertex, choose Vertex select(Ctrl Tab 1), select one vertex, press G to move and move it closer to another vertex, it will snap to it. Then make sure to remove doubles to avoid differences in weights
Here's a detailed video tutorial and a blend file Switch to orthographic ( NUMB 5 ) front ( NUMB 1 ) view. Select everything with A, Select - Select Boundary loop, Uvs/Shading - Edges - Sharp. Select everything again and Remove doubles with Merge Distance set to 0,0001. Now you see where the holes are. Select - Select More/Less - More, Shift H to isolate. Enable snapping to Vertex, press G and snap vertices. If there is no vertex to snap to, create it with Knife tool. If there are stray vertices or overlapping faces, delete them. Once done, reveal hidden with Alt H. Make sure to edit uv_0 and uv_1 maps, too. Use W - Weld to merge vertices. If it merges unwanted vertices, disable sync and Sticky selection and, holding Ctrl, lasso-select the ones you need and merge or snap. Once done, select one sharp edge, Select - Select similar - Sharpness to select the rest and Mesh - Edge - Edge split.
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