After some though, I've figured out a way to seal undeneath the cellular patterned A/C bottom pan. I can seal the very outside, preventing moist air from getting in, with Mortite!
I first became aware of Mortite more than 30 years ago reading (IIRC) The Absolute Sound magazine. Someone suggested this for reducing resonance in turntables by putting this under the sound board, the platter, or something like that. I obtained Mortite and didn't like it very much. It has a gritty texture, doesn't stick perfectly at first, then gets harder over time (which doesn't help its sound damping properties). Over time, however, it tends to become more rather than less attached to whatever its attached to, along with becoming harder, but never fully dries out. You can see the usefulness of this in sealing applications. Despite sticking pretty well over time, it's not permanent, so you can remove what went before if it gets damaged and apply anew (best done with a brand new box of Mortite).
And there's more. Like hard mud, you can pile it up from the bottom, and it won't sink down or fall over like flimsy tape. And it can be molded with finger pressure to the exact shape required.
What I will do for a belt-and-suspenders sealing application is first insert the extra open cell highly compressible polyurethane foam strip (provided by Friedrick for window application) into the opening and push it in far enough that 1/8 or so on the trim outside is available for piling on the Mortite. Then I'll pile on the mortite, and seal the outside with mortite so there is no exposed airgap. On the inside, I'll also stuff the remaining part of the polyurethane foam strip (I think there will be enough for front and back) and possibly put Tyvek tape from the trim frame to the bottom in front. So vapor transmission will be mostly through vapor permeable barriers in front toward the generally dryer inside, with the outside sealed against moisture, airflow, and everything (there shouldn't be water, since this is under the A/C chassis, but could be in extreme storm conditions) as much as possible.
One extra little trick might be to put small low voltage DC LED bulb underneath the AC with wire leading to the front. A battery connected to the wire would light up the area, exposing any gaps in the mortite coating, if viewed on a moonless night. I could do that check seasonally to be sure no gaps have opened up.
I first became aware of Mortite more than 30 years ago reading (IIRC) The Absolute Sound magazine. Someone suggested this for reducing resonance in turntables by putting this under the sound board, the platter, or something like that. I obtained Mortite and didn't like it very much. It has a gritty texture, doesn't stick perfectly at first, then gets harder over time (which doesn't help its sound damping properties). Over time, however, it tends to become more rather than less attached to whatever its attached to, along with becoming harder, but never fully dries out. You can see the usefulness of this in sealing applications. Despite sticking pretty well over time, it's not permanent, so you can remove what went before if it gets damaged and apply anew (best done with a brand new box of Mortite).
And there's more. Like hard mud, you can pile it up from the bottom, and it won't sink down or fall over like flimsy tape. And it can be molded with finger pressure to the exact shape required.
What I will do for a belt-and-suspenders sealing application is first insert the extra open cell highly compressible polyurethane foam strip (provided by Friedrick for window application) into the opening and push it in far enough that 1/8 or so on the trim outside is available for piling on the Mortite. Then I'll pile on the mortite, and seal the outside with mortite so there is no exposed airgap. On the inside, I'll also stuff the remaining part of the polyurethane foam strip (I think there will be enough for front and back) and possibly put Tyvek tape from the trim frame to the bottom in front. So vapor transmission will be mostly through vapor permeable barriers in front toward the generally dryer inside, with the outside sealed against moisture, airflow, and everything (there shouldn't be water, since this is under the A/C chassis, but could be in extreme storm conditions) as much as possible.
One extra little trick might be to put small low voltage DC LED bulb underneath the AC with wire leading to the front. A battery connected to the wire would light up the area, exposing any gaps in the mortite coating, if viewed on a moonless night. I could do that check seasonally to be sure no gaps have opened up.
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