Reducing Draw Weight on a Laminated Longbow

[Camp Creek]

I finally got back into hunting with a longbow I made over 30 years ago.  It's 70# at 31", and I'm seriously thinking about trying to reduce that to mid 60's or below.  I've watched some youtube videos and read some posts here about sanding the fiberglass, and that now seems the best way to go.  As near as I can tell with calipers, I have .060" glass on the back and belly, so I believe I have plenty to work with.
Using Virtual Bow and published data for the modulus of the wood layers, I can get the model to agree pretty well with the actual draw force curve by using a fiberglass modulus of 5,000,000psi.  This same model shows a drop to 61lbs at 31" if I reduce the back fiberglass layer to .050", and still have max compressive stress of just over 30,000psi which should be well within the safety limits for the fiberglass from what I've found on this forum.
My plan is to start with 80 or 120 grit on a drum and make equal number of light passes on the back of each limb, checking the tiller as I go, until I get below 65lbs then finish sanding with finer grits before refinishing with spar polyurethane.  I'll probably add some tip overlays as there are none now, and I'd like to look at lower stretch string materials.  It's clear fiberglass over walnut, so I'd like to keep as much of that look as possible.  Would 320grit be a fine enough finish?
I'm wanting to hunt deer, and the occasional pig or turkey, with a bow I made and enjoy shooting.  I'm not looking to squeeze every last fps out of it or anything.  Although, I would like to get get into building bows again at some point if I can some how convince my wife that I need another hobby...

Any thoughts, advice or experience would be appreciated.

Thanks

[Crooked Stic]

If you are using  a power sander 120 would be my choice. No need to go past 220 and some stop with 180 griit

[dbeaver]

Go for it.  At this point all the thinking it done the bow just needs material removed in an even manner counts your strokes on the drum or use a flat block and clamp the bow up good so you can get passes with even pressureand wrap your fingers around the bow limb so you can get the feel that its even pressure.  If you use a drum I suggest a pneumatic sleeve style where you can modulate the pressure of the sleeve to slightly round over the edges and so you are less prone to digging in on one side as the pressure will distribute itself a bit.  There are a few ways to reduce weight significantly and glass is a great option.  There are older threads here discussing the efficiencies of trapping the bow in order to reduce weight and just recently KennyM posted a thread about reducing weight doing just that I believe. 

[VTer]

I reduced the width on a bow recently and dropped 10# off it. Lost another pound tillering it by sanding the glass. I’m no expert but I was trying something

[dbeaver]

I also had done that when I first started I took a biw that was 1.5 at fades and it's now just barely over an inch and tapers to 3/8s...Went from 90's to a now very tolerable 55.  I did this when I had less tools and less knowledge and I thought it was my only option. I'm now going to make more like it because I like how it shoots lasers. 

[Kirkll]

A good rule of thumb for sanding glass is about .002  of glass sanding equals 1 pound. So to drop that baby 10# by just sanding glass you’d have to sand .010 off both the belly and back… I’d recommend a combo of narrowing the width profile, and trapping the belly side, along with sanding glass. I use a random orbital palm sander and start out with 80 grit for a reduction like that….

I’m not into all the engineering specs on modulus. But I know a 60# bow can safely be built with 040 glass. Even with some sanding for tiller adjustment….

With that being said, I’d never do a bow reduction on a bow I didn’t build myself from scratch, and even then, only why I was building it. I’ve had guys I built bows for years ago as for a weight reduction, and I had to pass on it. Hell…. How am I suposed to remember how much I sanded the glass when I built it 10 years ago?