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Hi guys, I’ve been offered an original fuel tank for a Z50 K1, inside is pretty clean however there are a few holes around the base that would need repair, I’ve attached pics of what they look like - any suggestions on methods to repair the tank would be greatly appreciated!
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The rust holes that you can see from the outside, are usually just the tip of the iceberg. The iceberg is inside of the tank, and the tip is poking thru to the outside. The trick is to determine where the bad, thin metal ends...then cut it all out. Then cut a patch to fit...and/or, cut the hole to fit the patch...then weld it in...as perfect as possible. It's pretty tough to weld such thing metal with a wire feed welder without burning holes. You may be able to get your patch welded in place with some tac welds. Then dress them down a little. Then braze everything to hold fuel...it's not necessarily a easy job. Certainly not easy for the average, handy guy.
After you get the tank back to mostly strong solid metal...maybe only a few drippy leaks...you can derust the inside of the tank with phosphoric acid. Then use a good tank sealer. Then you will hopefully only need a very thin coat of body filler...or epoxy putty to smooth out the outside for paint.

Don't seal the inside until you're all done heating/grinding on the outside.
Don't paint the outside until you're done with the sealer.

For a crappy temporary fix...
You can clean the outside of the tank to bare metal. Then you should be able to SEE all of the rust holes. Then you can tap them in...to make a low spot...to fill with epoxy putty. You can get epoxy that comes in a stick. You mix it in your hand and it's stiff enough that you can flatten it into a thin sheet...lay it on your tank over the hole...then stick it to the good metal around the perimeter of the hole. Give it TWO days to harden and cure. Then smooth it out a bit by sanding.
Then, you may still need to use a sealer inside if the tank.
Some sealers need clean metal to work properly. Other sealers can be used in a somewhat rusty tank.

Phosphoric acid WILL remove...eat all of the rust if you can submerge the tank in the acid. A day for lighter rust..maybe 2 or 3 for heavy rust.

Good luck.
 

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If your tank happens to be a K2, oval badge. I have a few for sale that are good...or at least better than the one you have there. My prices would be in line with the "going rate"... Based on...less than eBay.
 

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I had similar rust through on a K2 I picked up. The tank got blasted clean and then I poked in on the spots with an awl.



Then I was able to tig weld them closed and grind flat. Not perfect, but I am a novice welder.



It of course that was not the only spot. But it was basically rinse and repeat. The tank got a coat of primer and filled up to leak test. No leaks so far. As Kirrbby points out probably best to line the tank with one of the many purpose made products afterwards before painting anyways. Davmo has an excellent thread where he explains his process.


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Here is one...


And another...


BTW Thanks for being awesome Davmo
You too Kirrbby!


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Here is one...


And another...


BTW Thanks for being awesome Davmo
You too Kirrbby!


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Tanks, man...I mean thanks!
 

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I braze them with coated rod from depot and oxygen acetylene torch. When they hold air pressure. ( only test with few psi 10ish and soapy water) then coat inside with red-kote. Works great and I've fixed some rusted out tanks. I've had to break 1157 light bulbs to use brass base as patch panel. And with good heat control u can bridge pretty good gaps without adding metal.
 
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