Hi Peter;
  The tank is built into the fuselage and not removable? This is the main reason NOT to build in your tank on a full fuse model.  Fill the tank with some acetone and let stand for a while and lug up all the tubes. If you can find a way to rig a separate tank to the engine, do so in that you can run the engine on the ground for at l;east two or three minutes at a time. You are doing this to use the engine to create a sort of ultrasonic cleaner effect on the tank. Run it once or twice, then let it sit. run it a few times more, then flush the tank with clean acetone. If it comes out clear, do a test run one the ground, again, in short runs, like you are breaking in the engine. When the tank is empty, check the fuel filter and see what you caught. If there is anything in there, repeat the ground run process. Not as time consuming as it sounds, and it will shake loose anything in the tank that can be shook loose. I like to use white vinegar also, as it will attack rust also and has cleaning abilities on any sludge, and the engine vibration really is the secret.
  Type at you later,
  Dan McEntee

Since I have two more identical tanks, access via a hatch and silicone seal to hold it in, should not be difficult to do an R&R (remove and replace.)

I'm starting to think that tanks should routinely be cleaned BEFORE storage.  Some alcohol stove fuel may be the ticket.  Or vinegar or acetone.

Peter

Since I have two more identical tanks, access via a hatch and silicone seal to hold it in, should not be difficult to do an R&R (remove and replace.)

I'm starting to think that tanks should routinely be cleaned BEFORE storage.  Some alcohol stove fuel may be the ticket.  Or vinegar or acetone.

Peter

     Well, if you have a replacement tank, then there is the answer, and you can remove the back from the problem tank and clean it out. Just don't get too aggressive with tools and such as Lauri mentions to cut through the tin coating. If you get it clean enough to be reliable, then re-solder it using the rosin core solder mentioned and a minimum of flux applied to the outside of the tank only, test and flush well. Then when not flying it,just store it with some kind of fluid like stove fuel and make sure it is completely full, then cap it off. I used to use fuel, until these nitro situation arose and the quality of nitro in anyone's fuel is at question. Everything that Lauri mentions about building a tank is spot on, especially the brass tubing. Put a short scrap of brass tubing in some fuel in a baby food jar and let it sit, and watch what happens. Years ago at the hobby shop I worked part time at, we sold K&B fuel, and then Red Max hit the market at a cheaper price. They were the first to use imported nitro I think, and we had all sorts of guys complaining about how much trouble they were having with engines,tanks and such. More often than not,  we found that they were using Red Max fuel. We kept a display of brass tubing/fuel in baby food jars like I described so customers could see the difference. That was when there was nitro production in this country, and these days it is all imported and has an acid effect on anything ferrous like bearings, cranks and the steel under the tin coating on tanks and can attack brass over time.
    I come from a back ground where there wasn't a lot of money to go around and I had to make do with what I had and repair things where ever possible. Old habits are hard to break and I still spend time fixing something that I really should just throw away. Sometimes, fixing what you got can be the only option. I came up with the cockeyed method of cleaning the tank out of necessity in recent history. I came into possession of a Fierce Arrow stunt model that was built by and belonged to Mike Gretz.  I considered Mike to be a good friend and the airplane is pretty special to me. It is finished in silk and SIG dope as you might expect and powered with a Fox .35. When examining the nose of the airplane, there was only an over flow tube sticking out. I figured Mike just filled it through the feed line. I do not own ANY hanger queens in my house, and fully intended to fly this airplane every once in a while just in Mike's memory. I flew it carefully a few times to feel it out and check run times and such, but ran into issues like Peter described, and found rust flakes in the fuel filter. Mike built the tank into the nose of the fuselage, and no way to access it, and I sure as hell wasn't going to cut into the fuselage to try and figure a way to get it out. While sitting there staring at the nose, it hit me that the engine vibration is a pretty fine frequency, and that's what was breaking the rust loose. Desperate times call for desperate measures, and that is when I came up with the method I describe in my first post. It has worked like a charm, and I just store the model with the tank filled like I described. Stove fuel is pretty cheap at Wal-Mart and will protect the inside of the tank. When I flush out the tank to fly the airplane, I put one ground run on it so that first tank of fuel will flush out anything that was left in the tank, check the fuel filter, and then fly with some confidence things will be OK and remember the fun times I had hanging out with Mike Gretz at the SIG contest.  Sometimes, you just can't do what is the most obvious thing to do to correct a problem, and you HAVE to use what you have in front of you. I have had other old models I got from various places with built in tanks and since the airplanes had more problems than they were worth, I stripped off what I could use like engines, vintage wheels and hardware, and then give the air frame away or give it a Viking funeral. But this Fierce Arrow was not in this category, and I thought Peter's model might be like Fierce Arrow. It's like the old saying says, sometimes a man has got to do what a man has got to do!
  HAPPY ST. PATRICK"S DAY TO ALL!
  Dan McEntee

I did replace the tank with another one, subject to leak testing with air and final silicone adhesive.

Huge quantities of junk continue to come out of the old tank, even after several acetone flush attempts.  And, on top of that, I realize that I knocked a hole into the old tank with a cowl attachment screw. 

The new tank, which is also larger, passed the acetone flush test.  Also, the new tank is set further back from the nose, so the attachment screws miss it.

Not sure how I would implement a "store it full of stove fuel" procedure, as attaching the feed line to the engine can be difficult with a box fuse.  For a profile, easy enough.  I do agree that fuel tank construction closely resembles the quart fuel cans from the hobby shop, that storing full is not a bad idea.

Thanks for all the advice,

Peter