Realizing there are at least two ways to interpret the post title, i'll leave one and deal with the other. When reaming out a two layer cork butt, one that has rubberized cork at the very bottom, i'm finding it difficult to enlarge the hole in the rubberized section. It mostly just compresses when the reamer reaches that far down, and i don't want to blow out the cork trying. I'm also worried if i use a smaller diameter reamer that i'll end up enlarging the hole in the top layer of cork. (Hope i'm explaining my situation clearly)

So what do you y'all usually do? Is there a better technique for reaming out the center? Or do you just ream out the top layer and let the rod butt stop when it reaches the bottom, rubberized section? If so, do you trim the blank to meet the desired length, say 7' exactly? Thanks in advance.

Kevin, hand reamer or power reams? Power reamer will take care of that problem as it is easy to keep that type of reamer centered via the hand that is hold the cork. Power reamer seems to do a better job taking off rubberied cork also.
Buzz

I don't ream out the rubberized section. I ream out the cork section to fit the butt of the blank down to the rubberized section then insert the butt section so that it goes into the cork section but butts up against the rubberized section. If you ream all the way through, you end up with a big hole to fill at the end.

The butt can be easily filled With either a butt cap from auto interior or even a quarter glued on Easier to ream all the way though

Bill - willierods.com

Because the rod butt is generally larger than the portion upwards, it it best to ream from the butt end to obtain the proper fit to the rod blank. Bill makes a good point about boring/reaming all the way through and then filling the open end bore with something.

............

It just makes making a handle a lot faster and easier to do
Unless you make the handle and then get some kind of butt to fit Many different ones on the market

Bill - willierods.com

Kevin,
I have used premade grips in the past that used a center section of a type of rubber that was inserted into a punky material that was then finished with a layer of cork.

Here was a grip that was very very difficult to bore. The idea of the center rubber section is that for using the grip on blanks that were appropriate for the grip, one could simply apply a layer of glue to the blank and slip the grip on the blank and the grip would be finished. But, if one wanted to use that grip on a blank that was much larger than the rubber center section, it was necessary to drill out the center section without destroying the grip. I found that by using extended length drill bits long enough for the full length of the grip and using a variable speed reversible drill, and frequently backing out the drill bit, the rubber center section hole could be both enlarged, or if necessary completely removed.

When boring grips that have a finished butt cap, and you do not want to bore through the finished butt cap, I use a needle nose blow gun hooked to the air compressor air tank to frequently blow the drill shavings and dust out of the bore hole as I am cleaning out the bore hole. If one does not frequently remove the dust and drill shavings frequently, the shavings will eventually jam the bore hole and potentially ruin the blank by grabbing the inside of the bore hole while drilling and breaking the grip. But frequent removal of the shavings, by blowing from the bottom of the hole, works well to avoid issues.

Be safe