Searching for the best method to attach aluminum nerf bars (3 in round Zee Dee) to frame of 85 RC ??
I understand a little, enough to know that you should not bolt or weld aluminum to the steel,..
 per google search :::    "This is called galvanic corrosion. When you join aluminum and steel together, the aluminum will be preferentially corroded. ... Use an isolating coating or paint on the aluminum and the steel to isolate them electrically. However, all of your effort will go for nothing if you just bolt the joint together.Nov 19, 2014" 
Anyone who has done this, please advise..   TY.. Ed   

Can they be bolted to the frame with a gasket between the two different metals?
Jim

one of those "plastic" cutting boards that's used in the kitchen that's roughly 1/4" thick maybe a little less comes to mind...that is if nothing else is laying around the house/garage that could be used. Draw out a stencil and cut your shape

dodge82273
 jungle
Elwenil
flyfish

 Thank you all for your kind attention to my question. I poured over the internet, at least you can pretty much exhaust what there is "information wise" out there if you spend a few hours and use the correct search terms etc etc..  It seems that the most people who deal with joining two "dissimiliar metals" are people who build trailers and other tow able wheeled vehicles and pleasure craft boat builders. Both are concerned with weight and often use aluminum but still need to adapt existing vehicle axles which are steel, etc etc..  The boat guys stress to use the same type of metal e.g. aluminum rivets (never steel) when fastening two pieces of aluminum, say for a boat dash gauge cluster. I will  add..  links to the various articles that I found useful, to begin to understand how to approach this project. It's going to be a couple of months ... as I am finishing up hand wiring my entire RC.. part of which had to do with finding a digital voltmeter.... to partially compensate for the problematic ammeter gauge.  I thought that I was just going to have a welding shop weld the 3 in round black step bar.. (Nerf Bars) to the frame under each door to the cab. Basically what I have concluded is that.
 
(1) the mounting flange onto which each side of the step bar will bolt to, must be insulated from the frame.
(2) Yes a good coat of paint to each surface is a must, (but alone it's no where near sufficient) in addition to sandwiching an insulator, a non conductor such as a piece of rubber, or sheet of rigid fiberglass between the frame and the mounting bracket / pad..
(3) The use of galvanized / zinc coated carriage bolts are required, BUT where they pass through the other side of the frame, they must be insulated from contact with the aluminum mounting bracket / pad as they protrude through the mounting holes. Some use nylon sleeves. I am thinking that sections of small PVC pipe may work, or small nylon bushings, found in those pull out drawers of fasteners at the home improvement stores.
4) Not to mention a nylon washer to insulate the nut that will be tightened down the threads of the mounting bolt up against the face of the mounting bracket / pad. 
There may be a (5) and a (6) to this assembly.  You would think that someone would have fleshed all of this out ? How many trucks have aluminum frames   I will call the step bar manufacturer to see if they have a mounting kit, to save time and trial and error.  Had I known re galvanic corrosion, I may have had some 3 in round steel bars bent and just welded to the frame !!!!  The above is incomplete, anyone wanting to do this double check their own research.. This is load bearing stuff vs cosmetics... 

I forgot a major item, moisture, even humidity etc is the link between the two metals, it carries the current !!!   

P.S. That plastic cutting board idea is spot on, the wife got one from bed bath and beyond ....heh heh heh.....

    https://galvanizeit.org/education-and-resources/resources/technical-faq-dr-galv/using-paint-as-an-electrical-insulator-in-dissimilar-metals

   https://www.google.com/search?q=how+to+prevent+galvanic+corrosion+between+aluminum+and+steel&sa=X&ved=0ahUKEwiIh-Kfs5vdAhUPhuAKHThEDuMQ1QIIugEoAg&biw=1600&bih=743

https://www.hunker.com/12502151/screws-compatible-with-aluminum

https://www.westmarine.com/buy/tef-gel--corrosion-eliminator-and-anti-seize-lubricant-tube-with-applicator-20cc--12222212?recordNum=1

https://www.eng-tips.com/viewthread.cfm?qid=94941

https://www.albanycountyfasteners.com/Carriage-Bolts-and-Timber-Bolts-s/281.htm
     

We've got probably 7 of those cutting boards and I was thinking just the other day..."why the hell do we need 7 cutting boards"...that's what made me think of it LOL-and the material is really tough. Problem is my wife thinks she needs all 7 .   I have a thrift store close to home with all sorts of things I like to find problem solving pieces like this. A cutting board there might cost a dollar.

 
P.S. That plastic cutting board idea is spot on, the wife got one from bed bath and beyond ....heh heh heh.....
   

I'm glad I read your thread...now I'm thinking I will employ some of these ideas for installing my aluminum fuel tank. It probably does take awhile for corrosion of steel to aluminum to take place but with all that weight of fuel shifting around, a little corrosion might lead to a failure. IF you're lucky to have one near you, Ace hardware has a better selection of all those bushings, sleeves, etc than the big box stores like HD lowes, etc.