My understanding (maybe incorrect) is that all patinas on steel are a form of rust. By that I mean if not coated from the elements, the patina will eventually turn to rust, and it will do so faster than the plain steel next to it. I have had a lot of luck preserving these patinas for indoor use using a basic satin from Krylon or Rustoleum.
But outdoors what appears to be happening is the UV from the sun bakes off the protection from the Home Depot products, leaving the rust to have a field day. Outdoor steel art that were in a more shaded area seem to retain a decent level of protection from rust. However, it seems that the more extreme the environment (water, UV, etc) the greater the necessary protection, and the more the protection will change the look of the patina.
For example, a light dusting of Krylon satin for indoor use was fine and changed the patina very little, however the exposure is very limited. Several coats of Rustoleum gloss protects *ok* for outdoor use, but changes the lighter patinas a lot. The thicker and "wetter" the sealer, the more the lighter colors are lost. I don't see this as a fault of the sealer as much as a fact of light diffusion, diffraction, etc.
I also recently used Ever-Clear. It is a "two part acrylic urethane" where you mix a portion of part A into part B, and you have 6 hours to do all your coats. Using it was pretty easy, as long as you can plan all your coats and amounts. It isn't always easy to know how much you will need beforehand, but it seems to go farther than you'd think. After 2-3 coats, the protection seemed quite thick. I have high hopes it will protect from sun, wind, and rain for quite some time. However, the affect on the patina was pretty severe.
You can see in the two attachments how the patinas changed, as if they were held underwater. For the green/blue patina (on copper/bronze paint) the effect was very nice. On bare steel, the effect was fine, difficult to notice. But on the lighter patina (rust) the lights were lost to the darks. Again, this is not unexpected due to the coverage thickness. Where the Ever-Clear was thinner, the change in patina was less severe.
I would also like to try Clear Guard (which I understand is similar to Permalac) to see how it does or does not change the patinas, but my guess is that outdoor protection is directly correlated to how much the patina (especially light colors) are affected. I read a lot of complaints that Permalac didn't end up protecting outdoors for very long. But I would be interested to read other's experiences with protecting patinas, while also retaining them.