You can load a normal map like a diffuse map in 3D coat, then you can paint over it to clean certain parts. There is always some baking errors
No, no, no. A properly baked normal map will not have seams like this and you should never paint on a normal map unless you absolutely know what you're doing. Can't believe "paint out the seams" is alive and well in 2017.
OP, you need to ensure that the program you are using the bake the object has the same tangent basis as the program you are rendering in. Baking normals in Zbrush is not a good idea, as afaik it uses its own tangent basis (anyone who has more insight here could add to this). Essentially you are using one math formula to compute the normal map, but marmoset expects another called MikkT.
You can bake with MikkT in programs such as xNormal, the Allegorithmic suite, Quixel suite, Handplane, Knald, and more.
Toolbag 3 has a great baking toolset. @somedoggy is correct. ZB is not a good baking option. Something like TB3 will yield far superior results and uses the most modern tangent basis (mikkt)
Painting on normal maps is terrible advice. It is encoded data and is not like painting on other textures. Baking is a systematic set of rules that, if followed and understood, will not produce any errors.
In Marmoset Toolbag you can change on the model from what software the model and normal maps came from. I don't have Marmoset on my hands right now, but if you select your model in the left panel and hit the plus icon it will show another object below the parent. Click on it and you should see a roll up menu. There should be something like Marmoset, 3D Max, etc. Try all of the options and see if it solves your issue.
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OP, you need to ensure that the program you are using the bake the object has the same tangent basis as the program you are rendering in. Baking normals in Zbrush is not a good idea, as afaik it uses its own tangent basis (anyone who has more insight here could add to this). Essentially you are using one math formula to compute the normal map, but marmoset expects another called MikkT.
You can bake with MikkT in programs such as xNormal, the Allegorithmic suite, Quixel suite, Handplane, Knald, and more.
Take a look through this as it is relevant to all programs.
https://www.marmoset.co/posts/toolbag-baking-tutorial/
Painting on normal maps is terrible advice. It is encoded data and is not like painting on other textures. Baking is a systematic set of rules that, if followed and understood, will not produce any errors.
Petr