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GS850G Fuel tank clean up

  • Thread starter Thread starter jdvorchak
  • Start date Start date
J

jdvorchak

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Let me start by saying I hate to clean rusty, varnished up fuel tanks. Of all that I do to these bikes I hate cleaning rusty tanks. I'm no good at it and while I get them clean I hate doing it. Did I mention that I don't like to clean tanks?

I bought a couple of 850 in December and one the tank looked like new inside. This one? Pretty much looked like an old sewer pipe.



Since I had plenty of time, over the winter months, I just filled it with Diesel Fuel and left it. I was hoping that would break up the varnish and melt it away. That didn't happen. So I poured the diesel into my parts cleaner. It works good in there. So I filled it up with gasoline and let it sit a couple of weeks. Actually that did a better job of melting the varnish. Well I'm done with the engine/carb/electrical work so I forced myself to work on that tank. Drained the gasoline out into the parts washer. Couldn't hurt and I keep the lid closed on the parts cleaner when not in use. Plenty of nasty looking stuff came out. I then threw a handful of nuts and bolts in there and one gallon of plain white vinegar. Shook it up as long as my old arms could shake it over a couple of days. Put two more gallons of vinegar in there and shook and rotated how it sat. Well long story short I drained the vinegar and fished out the nuts and bolts with a magnet on a stick and flushed it numerous times with my garden hose. Kept that up for a while until I didn't see anything but water coming out and it looked clean. Dried it with my shop vac and poking a towel in there on a stick.

So my advice is to never buy a bike with a rusty tank but if you do vinegar and nuts and bolts will clean one. Just takes a couple of days of soaking.

 
Nicely done, and I bet that didn't take near as long as you thought! ;) What are your plans for sealing it?
 
Definitely use a sealer. I did the same as you, and didn't use a sealer at first. All was fine for a few months, and then a couple pin holes opened up. I used Caswell tank sealer after that. It's good stuff.
 
I just did one tank with vinegar after doing a dozen or so with electrolysis. It's a lot quicker, but the tank wants to rust up again right away. I'm thinking maybe combine the two techniques, maybe use vinegar to get rid of most of the rust, then go to the electrolysis to get the rest, then maybe add antifreeze to the water to let it's anti corrosion ingredients keep anything from rusting until it goes on the bike and gets filled with gasoline. Not sure how well that would work, but I have a collection of useless L tanks to practice on.

I'm in agreement about buying bikes with rusty tanks, but sometimes there's just no choice. Like the old R80ST I just got for nearly nothing, nice looking tank but a lot of rust and even some through holes already there. If it were a GS tank I'd just toss it and find another. It's not.
Replacement tanks for this old Beemer are closer to $1000 than $100, the bike in pristine condition is only worth about $3000 or so, that idea is out. Other parts are expensive too, a stock restoration just isn't going to happen. I could make a lot by parting it out, but that's not my intention at this point. Not sure how I'll proceed with this one, maybe some kind of cafeish dirt bike scrambler sort of thing?
 
Phosphoric acid is the stuff. Less flash rusting, but not perfect. At any rate, it will eat all the remaining rust off in just a couple hours.

Home Depot sells this stuff for $15/gallon. The pros use phosphoric acid when derusting steel, not vinegar.

6dfa3a82-2631-469a-ad51-bb6f7c88611f_400.jpg
 
There was more varnish than any real rusty spots so I'm not going to seal it. As for the flash rust, yeah the nuts and bolts were flashed by the time I got the tank mounted on the bike. I filled it with 87 octane gasoline and no flash rust 8+ hours later. I had thought that if I didn't have time to finish install and fill up, I would just run some diesel fuel in there, shake it up and put the gas cap back on. Then drain the diesel and fuel her up when time came. Any residual diesel would mix with the gasoline and will burn nicely.

As for phosphoric acid, I didn't have any. I had a couple of gallons of vinegar around the house which my wife uses to clean all kinds of stuff. Plus vinegar is $2.50 gallon.
 
tkent02 said:
I just did one tank with vinegar after doing a dozen or so with electrolysis. It's a lot quicker, but the tank wants to rust up again right away. I'm thinking maybe combine the two techniques, maybe use vinegar to get rid of most of the rust, then go to the electrolysis to get the rest, then maybe add antifreeze to the water to let it's anti corrosion ingredients keep anything from rusting until it goes on the bike and gets filled with gasoline. Not sure how well that would work, but I have a collection of useless L tanks to practice on.

I'm in agreement about buying bikes with rusty tanks, but sometimes there's just no choice. Like the old R80ST I just got for nearly nothing, nice looking tank but a lot of rust and even some through holes already there. If it were a GS tank I'd just toss it and find another. It's not.
Replacement tanks for this old Beemer are closer to $1000 than $100, the bike in pristine condition is only worth about $3000 or so, that idea is out. Other parts are expensive too, a stock restoration just isn't going to happen. I could make a lot by parting it out, but that's not my intention at this point. Not sure how I'll proceed with this one, maybe some kind of cafeish dirt bike scrambler sort of thing?
Click to expand...
If you go cafe you can pick a tank that you like the looks. Lots of cafe builders around here toss the stock tank anyway. Knock off Harley Sportster tanks are popular as are some the Honda Shadow tanks from the late 90's. I'm sure you can find something cheap that looks good. I always liked the looks of Triumph tanks. Like from a Bonne or Thruxton.

Here is one tank I did from my 83 CB1000C. It had 7 big dents in it and I filled them and ended up rattle can Rustoleum 2X flat black :

Before:



After:



only took one afternoon.
 
Last edited:
Couple more before and after:



Note clubman bars. Popular around here. I have them inverted but you have the option to run them so you are laying down.
 
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