Cleaning up a very messy UV map from Photoscan

Hi, I am somewhat new to Blender. I export UV textured models from Photoscan. See the attached screenshots. Photoscan creates a model of the house, then I bring the model into Blender to clean it up and add the square base. I then bring it back into Photscan, apply UV texture, and bring it back into Blender. My goal is to: paint the sides and bottom of the base brown to represent soil underground. I could do that by finding the faces on the UV map and coloring them in photoshop, but that seems painstaking. Also: I want to write text onto the front face. But I can’t do this, because on the texture map, the faces which make up the front face are all broken up. How can I select only these flat faces of the sides of the base, and get them onto their own texture map without them getting rearranged and broken apart? Thank you very much for any help.


You could use a copy of the object which you then delete materials, clean up a mesh (select all in edit mode and Delete - Limited dissolve helps), mark a new seams, UV unwrap on a newly created image and then use texture baking from original mesh to new model: Bake, Selected to Active.
You should have manageable texture then you can work on.

You can try to use texture paint in 3D view on original; orient faces you need aligned to screen and use Texture Paint - External, Quick Edit. You should set external editor in User Preferences, Files before. Make sure you save only your paint job from photo editor (other layers should be hidden before save image).
Back in Blender hit Apply button. You might want to increase resolution before QuickEdit.