Hi Tony, can you please post an example model for something that is not welding for you?

If you have 2 solids sitting next to one another, rather than 2 individual surfaces then you would not use join for that because join is for gluing unattached edges of surfaces together. Solids have all edges already attached to something so you usually need to do a boolean union to combine solids rather than Join.

But the specifics depend a lot on what you have for your model - so it is a bit hard to give you a definitive answer without seeing your actual model.

- Michael