my son has a really solid 90 truck cab. The only issue is the rain gutter lip above the windshield is rusty. Any good solutions to repair this? Birdtracker

Good question, hate to say it but "you're not alone on that island, Gilligan". My son had it on his 90 too probably 8 ish years ago. He ended up roughing it up and hand laying fiberglass on top of his, it was an old plow truck. I've seen many around here junked for just that reason.
It seems the later years of the D/W were worse for that plus rockers than the earlier ones.

the entire bottom of this one is like new. It seems water sat up in the rain gutter. Birdtracker

My 92 is the same. This happens on all the 72 to 93 trucks but seems more common on 81 to 93 regular cabs with the more square roofline.

There's no easy permanent fix. There's multiple layers of steel pinch welded and no access to the backside. Many folks will replace the entire roof, prepping the donor roof while it's off the vehicle (flood the thing with Ospho or expoxy)

You can drill out the spot welds and open it a layer at a time. Be prepared to fabricate as theres no replacement parts.

Or you could cut them off altogether. Wonder how much drag they have considered to completely shaved? Notice any new vehicles with them?

Still have to fix the rotted metal around the rain gutters, and no parts are available.

I'm in the same boat. Some of this stuff was still available 10-15 years ago but it is all pretty much dried up now.
I had no idea mine was as rusty as it was until went to take out the windshield and I cut the roof skin off.
It was starting to leak and the inner metal headliner part was starting to bubble pretty bad. Turns out there was nothing left of it. I've been rebuilding a piece at a time since I started in early July. Shaping the pieces one side or section at a time so it is easier to work with, using pretty much just a hammer and vice. Won't be 100% perfect but good enough for me. My crappy welds on the first couple of pieces were better after I changed out the flux core for gas ha. I started remaking the drip rail last weekend but forgot to take a few pics of it.

DId some more of the drip rail yesterday. It's a little wavy from beating on it and forming it by hand but it's pretty close and the trim snaps on ok to cover the imperfections in it. I was able to make it in a vise by folding the top lip over on itself, then using a a chisel I rounded off to form the bottom lip. I never did any rust repair this major before but it's coming along ok I think.

looks pretty good to me !
i'm no body man, but i have "beat" many inches of sheet metal into forms needed to repair tin work using the hammer/chisel/heat/beat method.
it might seem crude to some, but almost anyone can repair something if they put their mind to it.
i always say to myself : "there are persons out there WAY dumber than i am that can do this, so if that's true, i can do it as well !"...............

Just make sure the welds are water tight, or seal it with a good seam sealer across all the weld seams. Plastic filler is not water proof, if the seam is not made water tight, you will be getting another chance to practice you metal forming skills again in about a year or two.

I know where there (was) a 91 cab like that, perfect in every way except for that roof edge lip
Colorado sourced cab at that