Texturing models in Blender I a complete beginner in Belnder. (I use 2.8 version of it). I used to work with cinema 4d years ago, so I have the idea about some basics of working with 3D. But as I see Blender is a lot more complicated and complex. 

I found this model online and made used a Decimate modifier to reduce the size. Now I need to colorize it but for some reason I face this issue: 

After unfolding UV and painting the model it paints the inside part of it.



This happens when I zoom inside the model and paint it from the iside:



UPD.1
I figured out it has to do with Normals facing different directions, I recalculated them outside, it seemed to help a little but still it very edgy. 
There are some lines between the polygons and some of normals keep facing the wrong direction.
I believe some of your face normals are flipped OR you have overlapping faces / really rough topology, OR you need to unwrap your model more cleanly without UV distortion and maybe even increase the UV texture resolution.

To fix normals: Tab for Edit Mode, then A to select all. Ctrl+Shift+N to recalculate normals (untick "inside").

Fix overlapping geometry: a quick fix might be Re-Mesh modifier. Be sure to save an alternate version before flattening ("applying") a modifier because it is destructive.

UV Unwrap quick fix: Edit Mode, select all faces, U "Lightmap Pack" or "Smart UV Project". Make sure your UV image texture is a large enough resolution and that your UV islands have enough margins between to prevent overlap.