what thickness, gauge metals for floor replacement?

ok, i'm going to cut the floor on the xj. what gauge metal sheets should i use? 16g or 18g? the price i quote, 4x8 on 16g=$74, 18g=$62

how do i prevent the remaining of the floor from rusting? where to get por-15? other options?

you get por15 from www.por15.com non of the national chains like NAPA, autozone, pep boys etc. stock por15 products.

i don't know what guage metal you need i only had two small rust through spots above my leafs that were small enough to patch will expoxy putty. If the affected rust areas are small cut out the rust with a grinder and patch with epoxy putty ... for small rust spots like quarter size to narrow longer wholes like mine.

Be sure to spend some time on por15 site they have a whole line of products. To gap a larger hole they have a mesh for something 3X5" sized holes, a fiberglass trunk floor fiberglass mat patch kit a whole mess of nice products.

I used 16 gauge on my MJ, because my source was Lowe's and they had patch panels the right size in only two thicknesses: 20 gauge and 16 gauge. I'm happy with the 16 gauge but I think it's heavier than the original floor and for patching I believe 18 gauge would be adequate, as well as easier to work with. On the other hand, being a tad thicker the 16 gauge might be easier to weld -- I cheated and had a friend who is a welding pro do mine.

I can't imagine that you'd come anywhere close to using a 4x8 sheet. I had a very serious cancer that necessitated removal of a good portion of the passenger side from next to the emergency brake handle all the way back to the rear seat base. I used a scrap piece of 18 ga that was about 2x4 and had plenty left over.
Here's a very handy tip for you. If your hole(s) run up the tranny tunnel at all, you are facing some difficult curves to duplicate. My solution was to lay white paper over the hole(s) so that it laid flat to the floor and then trace the outline of the opening onto the paper by shining a bright light up at the bottom of the paper through the hole(s). This way, you are assured that your patterns will result in cut pieces that fit when bent to the correct contour. This technique is a miracle particularly where the tranny tunnel starts to narrow since you have a compound curve - from floor to tunnel and from wide tunnel to narrower tunnel.