Mystery adhesive on patch and how do I remove it?

[seismicfish]

It appears that there once was an AC unit on my rig, nothing now, but putting one in myself. Where the old AC was, there are three patches on the roof covering four holes. There are no rivets holding them on, just a thick (~3/16") layer of some sort of adhesive/sealant? (See photos) Inside, there is a bit of aluminum glued to the patch with a bit of wire to keep it from flying off (I guess). So, I can eliminate one patch with the hole I need to cut for the AC unit, another will be untouched, and the third hole (3" diameter) will have to be partially intersected by the hole. This makes me uncomfortable.

So, I am contemplating removing the old patches and patching them properly before putting the AC in. I have no idea what the adhesive is or how to remove it, or should I even bother? The adhesive you see between the roof and the patch in the second picture is quite translucent, very pale yellow, with a good amount of small bubbles in it. It his hard and rigid (relative to Vulkem or the ilk) but the point of a screwdriver can be pushed into it slightly with some force without cracking it, and it eventually seems to bounce back. I guess there are three main questions I have then:

-Should I bother repatching these properly?

-What is this stuff?

-How do I get it off?

[Melody Ranch]

What I would do....get a putty knife and peel the old patches off. Use an electric drill, air if you have it, and with a light grade wire brush, clean the surfaces off. Clean the entire area from just ahead of the air conditioners future position to the area behind it. Spray some zinc chromate, rattle can type will do, on the cleaned area. Now, cover the entire area with a new piece of aluminum....you can leave the A/C 14 x 14 hole or cut it out later. Use Vulcum or similar for a sealant around the edges. Drill, screw or rivet down (olympics) this new aluminum. You don't have to go overboard on the number used. Then, I mask off the area, with overlap of the seams of the new aluminum with a white roof sealant. I use the ceramic white type. This provides you with a clean, watertight, smooth surface underneath your new A/C. You can't see it at all from the ground level. Watch out for wiring when you drill the holes up there.

[seismicfish]

Not a bad idea. I may do that. What gauge aluminum sheet would you suggest?

[seismicfish]

And what size rivets are best for this application? 3/16"? How large of an area do you recommend for the mega-patch? In the first picture, you can see the lines of rivets along the lateral bracing. Do I need to overlap those or can I just go almost up to them? If so, then I would be looking at a sheet ~20x22". Spacing on rivets?

[Jammer]

Any adhesive can be removed using heat.

Lots of threads on rivet choice and spacing, use search.

[seismicfish]

Thanks for everyone's help. Below are the before and after pics for my first patch job on the AS. I just hope it doesn't leak. Used a whole tube of Tempro on it.

[J. Morgan]

I estimate the chances of leakage here between slim and none.

Nice work.