How do i reduce the poly count of a giant mesh?

all my attempts to reduce the poly count of giant million+ poly meshes have led to my computer freezing up and blender crashing. It has tools that pretend they can do this, but they just fail epic.

the only work around i’ve found is to hand paint a few “inches” at a time and then dissolve 100-1000 faces at a time, which can still take a while, but 2 minutes x 1000-10,000 for a million-10million face mesh is insanity. People are coming in with insanely dense (and largely undeserved density) scans but none of them are useful without a million dollar computer. Is there no better way?

one section of the last mesh i worked on was FLAT, and it had like 30,000 triangles! Please help stop this madness.

I can’t tell the actual purpose of your need. You are talking about reducing then talking about painting some areas. What are you exactly trying to achieve?

In any case, if you want to reduce polygons you can use external tools like Meshlab which is what I use when I have to reduce the poly counts of huge meshes.

You can use the Decimate modifier but to be honest the modifiers are not the best way to go for super dense meshes.

Check this addon and its retopology workflows:

take a look to:
Edit Mode -> select all (A) -> menu Mesh -> clean up
Probably the tool “Decimate Geometry” can help you more then the others, but you can try more of them