Render transparent objects?

I just finished a blender project and now want to render it, but it is too big to render all at once (20"x30" poster print at 300ppi [6000x9000pix] so I tried rendering in sections. Render settings are PNG RGBA Premultiplied so I can add a sky gradient background using GIMP.

The main object is a lighthouse. The light beam and halo are partially transparent and where several renders overlap slightly the color strength gets increased (see the 3 horizontal overlaps in image).

Is rendering the background in blender the only way to avoid this or is there another way which I haven’t thought of?

I tried using the Clone tool in GIMP to get rid of the horizontal lines, but no luck.



try cropping out the overlap?