What are the drawbacks (I assume many) of building a spinning rod off a fly blank - to be used for surf perch fishing (casting ~1-oz. weights)? Specifically, I'm entertaining using an older Dan Craft FT 8-wt. that has been gathering moss.

None necessarily, although most fast action fly rod blanks are not going to be as fast as a fast action spinning blank, if that's important to you.

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Don't think your blank will have the power you hope for, a medium power, 8 - 12 lb steelhead blank is in the 12 power range.

For 1-oz. weights you will need a 10 to 12 weight blank. Pick one, which has fast or extra fast action. It will never be F or XF spinning-type, but it should be ok if you can live with MF or M action.

My first build - over 45 years ago was on a 9ft Fenwick fiberglass fly blank. Still one of my favorite spinning rods. Largest landing with it was a 23lb King Salmon

As mentioned, there are "no drawbacks". I will say fly rod blanks tend to cost more though.

Your best bet for a budget build might be to research which of the fly rod blanks from the past were "wickedly fast" and see if you can locate them at a deep discount due to age. I have a 9 weight that I "think" is an old Rainshadow Xcel blank. IMHO it would make a nice 10-15# class surf rod and might handle 1 ounce.

Russ in Hollywood, FL.

I doubt anyone would be tempted into making a surf rod blank into a fly rod, but I did build a hot-shot blank into what turned out to be an 11wt. fly rod. It feels funny but it works, and it has whole lot of lifting power. Your fly-rod blank spinning rod will lack lifting power.