I have been asked to build an ultralight spinning rod with a foam core handle. I find much info on shimming a reel seat but nothing for shimming a handle
The foam handle blank has a .3/8' hole and is 10 3/4 " butt to reel seat
the rod bland is .26" diameter at the butt.

I have built several rods with foam core/graphite handles for this client & am building a casting rod for him now. The client likes the capability of coloring the handles & decorating them with contrasting drag marks.

From what I have read, masking tape shims are a no-no, I think I can install 1/2 to 3/4" wide shims made from fiberglass tape, pulling them down with thread & doing a minimum amount of hand sanding as needed for a snug fit with paste epoxy coating on and between the shims.

one "backdoor" idea that has crossed my mind is to install a cork handle and finish it as I would the foam core handle.

Any help and/or ideas on completing the rod as the client desires are appreciated greatly,

Hey, masking tape has done the job for many years/decades. Sure, these days you have better options. Still, for what you want to do, it is a fast and easy solution that has been proven to work.

Russ in Hollywood, FL.

I have found that dry wall tape works very well for such a situation. I like to tack the leading edge of the tape to the blank with a drop of super glue. This prevents the tape from ‘walking’ when epoxy is applied. After tacking, I tightly wrap the dry wall tape in place until I get the right fit. If it’s only a few wrap I’ll go ahead and apply epoxy and slip the component into place. If it is more than a few wraps, then I’ll unwrap the tape and then apply epoxy as I tightly rewrap the tape. This forces the epoxy into the mesh and gives a very strong bond. You can either slip the handle in place immediately, or let the epoxy cure, sand to fit and then glue the handle in place. Whatever, you are most comfortable doing. The dry wall tape is extremely light, because most of it is mesh (Air), and this mesh allows the epoxy to form a very strong bond between the blank and the component. Takes a little more effort than using masking tape, but in my opinion I like it better.
Norm

One trick I've done in the past when using an EVA grip with a 5/8" ID was to use reel seat arbors to support the grip section on a small diameter rod blank. The same thing could be done here. If using a reamer to bore out the ID, you may need to cut the core into 2 or 3 sections so the taper of the hole doesn't become a significant issue. The core material works easily enough to sand to smooth out any minor issues at the joints.

You can just turn some thin cord around the blank to built it up than slip on your grip.

I have rods ranging from ultra lights and fly rods to surf rods all using either string and epoxy or tape some 40 years old and never have had a failure. Small diameter string epoxied to blank light sand to get good fit and rough up and glue handle.

Don't overthink it.
In spite of words to the contrary, Masking tape has been used for many many years by thousands of builders with never a failure.

As with anything, if not done properly, there could be an issue.

For relatively thin shims or arbors - I commonly use Masking tape. I just use rows of masking tape arbors with a very thin dividing line between the rows of tape arbors. Then, when gluing up the grip, I just insure that epoxy completely encases each masking tape arbor and I have never ever had a tape shim fail when using this method.

Not difficult, and completely reliable.

Be safe

i have only done this on two rods, so far so good.and that is to use a few wraps of reinforced gorilla tape instead of masking tape..it,s much thicker and takes fewer wraps but it has to be deglossed with a 3m pad to help adhesion..it,s also water resistant..two wraps would probably fill the gap you have..

With drywall tape the glue permeates the tape right down to the blank. What could possibly be stronger?

Someone on this site posted about drywall tape, maybe 15 years ago. It’s the only shimming system I’ve used since (I’m not considering reaming those CFX seat inserts as shimming). It’s light, strong enough for Halibut rods, and easy to use.