Does anyone seal their fans to these AIO radiators?  I've got duct seal putty, duck tape, foam tape, foam, silicone, etc......what do you recommend?

Well, I think it's needed and am going to do it.  If no one suggests anything I'm going to use a mix of duct sealer and gorilla tape.

kevinc313
Well, I think it's needed and am going to do it.  If no one suggests anything I'm going to use a mix of duct sealer and gorilla tape.

Both will Peal off the Paint and Void your Warranty.

I would want to be able to switch out and clean the fan so I wouldn't use anything this extreme but it would be interesting to see the results for a before and after. Honestly though I think it's something that if it was a big difference computers would have been using something more extreme for the past 30 years.

Yeah sorry I didn't do a comparison.  I'm not going to undo the tape because it's a really clean job and took a while.  The duct seal squeezed through the cracks too much.  Saw the use of spacers, I'll believe they help fractionally, don't have space to fit.
 
It's actually not doing any significantly cooler than the dual fan NH-U9 that was on there, but I didn't extensively test that with Prime95.  The U9 did about 70C on the Intel processor diagnostic tool burst stress test (170W for 20 seconds), and the CLC 120 about 68C, best case from cool idle.  Reviews indicate they are pretty much in a similar performance class.  I'm guessing there is 2-4C advantage of push-pull fans vs. a single and 0C-1C sealing them.  Going put MX-4 TIM on and call it good.
 
The real win is that it is CRAZY quiet with those A12 fans in a pair, even at max rpm.  1700 rpm is really quiet, 1000 rpm nearly silent.   I bet the sealing helps that a bit.
 
Going to install another pair on a hybrid, the four will be the only fans in the machine, front intake, rear vent, 4U rack case. 

Update.  Ran a bunch more prime95 non-AVX (138w-142w) and also figured out how to run the Intel diagnostic tool prime stress test for extended time (168w).  Same parameters as yesterday.
 
- Started with prime95, seemed to top out at 72C today, or 2C cooler than yesterday
 
- Idle temps are about 27C on a fully cool machine and 37C immediately after a stress test
 
- Ran Intel diag stress tool until temp stabilized at 80C
 
- I've got a U.2 900p Optane drive mounted inches away from the rad that runs about 35-40C normally.  Booted from an m.2 with the Optane powered off, no change in prime temps
 
- Unplugged the inner pulling fan so only the push fan was running.  Prime95 stabilized at 79C, or a 7C increase.  Was seeing slightly higher package power readings, like 142w vs. 138w with both fans.  Obviously this is not a valid test, but it could give an idea of what kind of performance you'd get from one fan blowing into a slight obstruction.  Keep in mind I don't have any exhaust fans, just big vents.  Plugged the inner fan back in, whew.
 
- Re-pasted with MX-4, slightly smaller than pea size.  Didn't get a real clean install because one of the stud plates is falling down.  Small pea size. Once I got the cooler on loosely I moved it around in a small circle with even pressure to try to squeeze out any excess paste or air.  No change in temps.  Maybe once it cures a bit temps will improve slightly.
 
Final results: Intel tool for 10 minutes at 168W = 80C, Prime95 non-AVX 10 minutes at 138w = 72C.

7°C Increase is from not running in Push/Pull
There is no Cure Time for MX-4.

Happened to run Handbrake for about a half hour the other day at about 150w power, 100% load, 4.7 GHz sustained all core clocks - maxed out at 75C iirc. 
 
9900K on MSI z390, stock/auto.