Consider the grade of aluminum as well, for structural reasons, 6061-t6, 2024-t3 or 7075 are structural grades ideal for a floor, .120 may be overkill. I used a combination of .040 and .050 2024-T3 aluminum for my floor. I bought it surplus from a metal dealer near an airport for less than 3$ a lbs. These grades tend to corrode easier than other so treatment is necessary. My prep was de-oxidize with a maroon scotch-brite pad, wash with phosphoric acid, air dry, convert with chromic acid or a similar alodyne or irridite product, air dry, wipe clean with acetone then isopropyl alcohol and apply a good expoxy primer. I use Endura or PPG super koropon primer. Super K is the same stuff used on the space shuttle. I also used a light coasting of 2 component poly-sulfide sealant between the steel frame and the aluminum panels to slow down the corrosion process. A marine urethane sealant will also work well.
I used Cherry MS rivets Part no. CR9163-5-2 and 5-3. You do not need a special puller to install these, just match the tip size up with the mandrel, not to tight or it will break low. They break flush and can be shaved down if the grip range is not accurate. If you want the correct tool to pull these, then your looking at big dollars, or course eBay is a great source but you may need to learn how to rebuild these tools, not cheap if you need more than o rings.
If you go the cherrymax route, you can use an HF or any inexpensive puller to do these as well, I wouldn't want to do any more than a few -4's by hand. There's lots on ebay aswell, just make sure you buy "max" type rivets or MIL 7885 spec. The last 4 digits will start with a CR/AF"3xxx" and you see the driving anvil on the top, looks like a washer. The Last three digits de-note material, diameter group, and head style(universal, countersink, NAS1097 shear, and unisink). Many companies make them not just cherry, but also huck, allfast, and voi-shan. You can download the process manual at the Cherry aerospace website to decode the part numbers. Avoid any CR2000 series rivets, they require a special double action gun to install these. You can also buy new certified rivets from aircraft spruce, but for more money. I could ramble all day about aircraft fasteners, but I think Cherry MS is the most economical option next to the max type rivets. Another benefit of the MS, is that is does a better job of filling the hole than a max type rivet, which is just a really fancy clamp.
As far as non aerospace grade rivets, Cherry Q is good, but haven't seen any great selection of the -5 and -6 dia. stocked anywhere. Some of the other structural rivets like the monobolt and magna-lok start at the -6 dia. Monobolts need a special nose piece as well which is not available for the lesser quality pneumatic riveters, I know as I used to own one and could get it to work. If you go with a .120 thickness floor you'll need at least -6 dia rivets with much closer spacing and maybe a second staggered row.
As for the layout, a rivet spacing fan is a time saver. I wouldn't necessarily worry about getting an exact spacing, but something close enough but equal spacing between fasteners, the fan lets you do this easily. Cleco's are a must too, yardstore has a good deals, not the best quality, but for the amount your going tot use them, they'll do.
Here some some pics of some of my tools equipment that make life easier for doing hours and hours of drilling and riveting. The egg cup type thing holds the drill bushings, makes drilling perpendicular holes in thicker materials easier. De burring tools and the spacing fan also pictured. If you can afford it get a nice light air drill, 1/4 chuck is ideal along with a light air hose, you may be holding this thing for hours.
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