Countersinking GRP for skin fittings - how to?

[Plevier]

I'm replacing the skin fittings in a balsa cored hull.
The original fittings are flush faced countersunk but odd sizes. (Lots of figures here to get your head round, sorry!)
The 3/4" ones are 26mm thread dia, 35mm flange diameter, 2mm at the edge and 5mm at the root which is an included angle of about 110 deg. That's a very small overlap of the GRP and a sharp countersink angle.
The 1 1/4" ones are 42mm thread dia, 61mm flange dia, 2mm at the edge, 5mm at the root which is an included angle of about 145 deg - much flatter.
The new fittings from ASAP are:
3/4" - thread 26mm, 49mm flange dia, 2.5mm edge, 5mm root, included angle 154 deg.
1 1/4" - thread 42mm, flange 65mm, 2.5mm edge, 5mm root, included angle 154 deg.

The easy choice is to turn down the new fittings to match, fine for the big ones, but on the small ones I don't like the small overlap and sharp angle.
Pic attached of the smaller fitting with old fitting shape in red which may help!

So how can I do a countersink in the GRP centred on the existing 26mm hole, 50mm dia overall, about 150 deg included angle, and sunk in 3mm at the outer edge to take the new 3/4" fittings (3x) as is?

I really can't see a way without a special tool with a pilot shaft and a couple of blades.

Any brilliant ideas please?

 

[lw395]

Better to remove some strength from the fitting than the hull?
Unless the hull is solid at the through-hulls?
You may want to cut the core back and fill with a resin/glass/filler mix around the fitting.
You can do the counter sinking by gluing some coarse abrasive paper to the tapered face of the fitting and spinning it in the hole with a big drill.

 

[Plevier]

Better to remove some strength from the fitting than the hull?
I wouldn't be going any deeper than it is and spreading the load much better.

Unless the hull is solid at the through-hulls?
No. They just drilled through and smeared lightly with resin. Lousy job.

You may want to cut the core back and fill with a resin/glass/filler mix around the fitting.
Yes, intending to.

You can do the counter sinking by gluing some coarse abrasive paper to the tapered face of the fitting and spinning it in the hole with a big drill.
Genius! Yes I bet that would do it. Just need to work out how to connect to drill. Wooden plug maybe.

Thanks!

 

[bignick]

A way to hollow out the core between the skins is reputed to be a small Allen key in a power drill. You can then fill the void with a suitable waterproof (epoxy based) filler and then drill through again to give a good waterproof hole that won't allow water into the core.

 

[Sneaky Pete]

My first thought is to turn a wooden bung the diameter of the hole and profile of the countersink. It will require a spindle/shaft to attach to a power drill. Along the countersink profile clue sand paper to act as a cutter. Another thought is to use a machine router with a profile cutter. Depends on how much space is available and how flat the surface is.