Railing suppliers

https://www.stairsupplies.com/

https://atlantisrail.com/

Update: https://www.affordablestairparts.com/products/stainless-steel-cable-railing-kit?variant=31783160643666 sells a simpler system with 1/8" cable. 9 20' lengths would be $433.08 including all the tensioning hardware (which I didn't even get into with Stair Supplies).

Stair Supplies .com

These guys will solve my problem, I think. Railings from Home Depot or Lowes don't look good to me, and clean designs don't seem possible with the limited range they carry.

5/32" stainless steel cable. $300 for 400'

Start with the cable. A 36" railing with a 2x cap and a 4x stiffener under the cap means we have a 31" gap to fill with cable. Less if we have a bottom rail, but I don't want that. For the 4" min. gap requirement, we could use 7 cables, but Fine Homebuilding and others say space them tighter because they flex under load. That would be 9 or 10 cables.

100' spool of cable is $75, and 400' is $300, so 75¢ a foot both ways. The upper deck has a J-shaped railing 16'-2' long (7' 4", 5' 10", 3'). That's 114' of cable at 7, 162' at 10.

The cable is 1x19 (meaning 19 strands). 316 marine grade, high corrosion resistant. You can buy lesser cable.

If I go with Stair Supplies posts (not 4x4s), I have to use their cable spacing (3.125" on center), and 10 cables for a 36" high railing. Posts are spaced maximum 4', and it says the cable can make up to a 45-degree turn, so I need a solution for two 90-degree corners.

They have a $59 Cable Rail Installation Kit. It has two top quality tools I probably need and a bunch of junk--allen wrenches, a post level, Torx driver bits--that I already have. It's a clever pitch: You pay $159 and they refund $100 if you return the two tools in 90 days. They have a video.

All things being equal, I like side mount posts. They maximize the deck area and hide the connection from above. We want the Side-mount, Flat-top, 2" square powder-coated aluminum post for $130. Colors are $10 or $20 more. Black Fluoropolymer is $140 per post. I'm not sure how they handle corners, but they make an outside corner post for $145/$155. I'm looking at 4 normal and 2 outside corners--$810 in black, $870 in black fluoropolymer just for the upper deck posts. They sell 5/16" x 6" GRK mounting screws for $2.15 each (in packs of 4).

Finally, the handrail. The aluminum posts are screwed into a wooden handrail from below. Their 6000 railing is sold in 10' sections. 1-5/8" x 2-3/4" red oak for $12.82/LF. The Upper Deck has 16' 2", so 2 boards at $128.19 each is $256.38. The red oak comes with no finish--they offer finishes for more $. Turns out they have N woods, M finishes, and X lengths. I could get a 10' and an 8' for less waste, for example. I'm not sure what "plowed" means in this context. Planed?

Or, I can match the posts with black powder-coat aluminum railing for $22.65/LF. Not sure how that turns corners, but I'll bet they offer an expensive solution.

So, minimum prices:
6 posts - $810
6x4 post screws - $51.60
162' feet of cable (2 @ 100') - $150
Red Oak handrail (18' @ $12.82) - $230.76
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$1242.36 for 16' 2" of Upper Deck Railing, or $76.85 per foot.

I think I'd better think it out again.--Reviewing the Situation by Lionel Bart

So, how about those 4x4 posts? Dunn Lumber sells pressure treated 4x4x8 for $14.18. If I put two at each outside corner, I'd need 6 posts and two 2x4 intermediate cable supports. Three 4x4x8 and a little for the 2x4s is $45, rather than $810 for the posts.

If I use 2x4 stiffeners under the rail, I only need 9 cables, not 10. So, only 145' of cable, not 160'. Dunn Lumber sells 2x4 Ipe for $6.98/LF, but fulfills orders with random lengths.

That works out to:
6 posts + 2 intermediate 2x4s - $45, say $50
Special screws - allow $50
145' feet of cable (2 @ 100') - $150
Ipe handrail (18' @ $6.98) - $125.64
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$375.64 or $23.24 per foot.

4x4s are looking mighty attractive. Money left over to buy the drill press I'll need for drilling the 100s of precision holes I'll need.