I'm putting the new cafe K100 back together but I'm wondering, whilst the 33 year old tank is off, no parts in or on it but newly painted, should I seal  it to protect it from future leaks. UK is now on E10 petrol and the tank has not leaks or internal signs of corrosion.
Should I seal it now and risk damage to new paint ect?

UK is on E10 petrol. Although you can still get E5 at some petrol stations.

Pete Riley wrote:UK is on E10 petrol. Although you can still get E5 at some petrol stations.

Yes I'm aware. I've not seen any information on using it on K's or alloy tanks. So my question is do i need to apply a sealer to protect now or not?

If you don't leave stale fuel in the tank for a couple years I would guess that you shouldn't have to worry. 

Corrosion will only be the next owner's problem when they buy the bike after it's been out behind the shed for a couple winters.

If you're worried about rubber deterioration, take out a new mortgage and feed it on Super Unleaded. It's E5 by another and more expensive name.

BMW Technical Service Bulletin SB-00-021-07 (NHTSA 10030825)

BMW MOTORCYCLE: ENGINE FUELS CONTAINING ETHANOL. BMW MOTORRAD HAS NO OBJECTION TO THE USE OF SI ENGINE FUELS CONTAINING UP TO A MAXIMUM OF 10% ETHANOL AND IMPOSES NO WARRANTY RESTRICTIONS. ETHANOL MIXTURES IN PROPORTIONS HIGHER THAN 10% CAN HAVE AN ADVERSE EFFECT ON PERFORMANCE AND CAN RESULT IN DAMAGE TO COMPONENTS THAT COME INTO CONTRACT WITH FUEL. NO WARRANTY CLAIMS WHATSOEVER FOR DAMAGE CAUSED IN THIS WAY WILL BE ACCEPTED.

This applies to K bikes.

Here it is on BMW letterhead:

Thanks duck for the posts. I think given the fact that it's in good condition, freshly painted and I've got an XS750 1977 which will benefit from the sealer more, I will just rebuild and ride the K.

I will get some pictures up when finished.

Many thanks

RC

According to my long-term experience on BMW as a mechanic, about Ks specifically: the less ethanol there is, the better it is. 
So no ethanol is ideal, E5 is OK, problems start with E10.
These problems are: 
- fuel pump damper desintegration: ethanol melts it.
- fuel lines desintegration: the same. Inside fuel lines create unsolvable leaks as you can't see it inside the gas. Outside fuel lines will burn your bike.
- fuel pump death: by absorbing water when stocked a few months, ethanol creates a gel that sinks at the bottom of the tank. This gel is then sucked by the pump but it can't deal with its resistance and dies. The fuel filter is also concerned and will be clogged.
- I also strongly suspect ethanol to alter the fuel indicator seal.
- As the tank is in aluminium, corrosion seems not to be a concern. But the fuel tank cap suffers from it.

The less, the best...

BTW , do you know why we have this ethanol in the fuel?
No, it's not to save the planet or polar bears.
In 2017, European Union stopped the sugar production quotas, at the time the price of sugar was very high. 
As predicted by economists, every producer invested to raise their production (mostly from beets)...but buyers did not put 10 sugars in their coffee. Same demand for over production : the price of the sugar collapsed and thousands of tons of beets were let rotten in the field.
The solution to use this excess of sugar was to transform it to ethanol and to create a regulation for a mandatory part of ethanol into the fuel. 
Paint it green, lower taxes, it will sell.
Damage to fuel tanks is not the worst, after all. Beet culture relies on a large pesticids use , that decimates insects, particularly bees ... Beet lobby is so powerful here, it managed to derogate to neo-nicotinoids prohibition.

Well jbt, I'm impressed! Thanks for the update and information . I'm doing what I can to combat all of the above but I'm stuck in the same Ethanol saturated world as most, so I will try to be vigilant when it comes to storage etc. I may even use the odd tank of super unleaded to try to flush some sins away.
Wish me luck.

We here in the U.S. were ordered to use 10% ethanol fuel by Al Gore as a sop to Iowa corn farmers when he was running for the presidency in 2000.  It is supposed to add a couple oxygen atoms to the combustion process to reduce the CO(carbon monoxide) in the exhaust. Tens of thousands of small boats had their engines and fuel tanks destroyed by the stuff that formed in their tanks by combining with the ethanol.

Ethanol in the fuel can be lived with.  I use it in all my vehicles both 2 and 4 wheeled.  The key is to run them often enough to use the fuel in the tank in less than 2 months.  If it sits longer than that the problems will begin, and even then it takes another 6 months for the damage to begin. 

To avoid problems from winter storage, I switch over to non-ethanol fuel for the last couple weeks of the riding season which in my area is November.  This flushes the ethanol out of the system.  Then I put stabilizer and a fuel system cleaner in the tank for the winter. 

With this regime I have had absolutely no problems with rubber bits in the tank and the engine starts instantly in the Spring after 5 months of storage as if I last rode it yesterday.

Excellent points of note .75. Is an achievable plan and makes sense. Many thanks.

Point-Seven-five wrote:We here in the U.S. were ordered to use 10% ethanol fuel by Al Gore as a sop to Iowa corn farmers when he was running for the presidency in 2000.  

The U.S. E10 mandate was signed into law by George W Bush in 2005. The regulation was a boon to corn farmers for sure, but it also lowered US imports of oil from OPEC which was part of its intention. Gasohol has been used in the US since the late 1960. I was using Spur Oil Co.'s brand in my 1949 four-door, fastback Belair Chevy in 1970. It ran ok, for a Chevy.