Are handle kits built onto the blank or built on a shaft and then assembled to blank.
What would be the pro's and con's of each.

Bill, handle kits or handle components are generally always reamed to fit the blank and glued directly to the blank. It would be hard to find a shaft to match the taper of the blank to fasten the grip components to. A separate shaft would add weight and could conceiveably cause a stress riser where the shaft meets the blank causing breakage.

Most rods were once built with separate handles which then either glued to the blank or attached with a collect and butt ferrule. It's still an acceptable way to do things.

The butt of most blanks generally aren't very flexible to begin with and most will easily withstand the attachment of a separate handle assembly. Mud Hole even offers such handle kits which you then affix to the butt of the rod blank.

Most rods these days, however, are built straight-thru with all the components fitted and adhered directly to the rod blank.

There are situations where you may wish to achieve a faster action or gain some additional length and this sort of situation is where building the handle or a portion of it on an extension piece are good options.

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I build all mine handles with cork or whatever material I am using off the rod on mandrels and then finish reaming out the cork to fit on the blank. Then glue up everything onto the rod. I do not like gluing up the handle on the blank first then turning it on the rod, I like turning my handles seperate on a lathe.

I echo what Mike says.
It is much easier to correct a mistake if the cork is only on a mandrel.
Same goes with reaming arbors, we always ream first to fit the blank, then glue into the reel seat.

Best wishes,
Ron Schneider
Schneider's Rod Shop
Mountain Home, Arkansas
[www.schneidersrods.com]
mtnron40@yahoo.com
870-424-3381