Caulking the tender.

With the large number or screws with fine threads required to pull the tender sides togeather, I suspect getting 100% metal to metal contact will be difficult
I am very tempted to run a soft solder seam around the joints in the tender. ( I have a very large high wattage iron which should have sufficient heat)
Has any one tried to caulk the seams this way?

The rivets, I found pulled it in close enough,You could use a bit of J B Weld as a filler if needed.I think the soldering is a bit overkill,but it is your Project.
Steve H

If you are going to use Isoflex to seal the tender then that is thick enough for any minor gaps, the metal is quite soft and when all the rivets are in fairly tight you can tap it a bit closer then tighten up the rivet bolts again as they will have gone a bit slack. With Isoflex make sure you degrease the metal first and then apply three coats fairly close together (so they bond as one) and an extra dollop on the nuts so they virtually disappear in the gloop. If rust is going to start anywhere it will be around the fixings. My 4" tender is 10 years old now and after a 'good looking at' this Autumn there was no deterioration or rust anywhere inside.

I tried soldering my 4" tender, main problem with this method is heat. Even using plumbing solder with water based flux it's very hard to get a good flow, I was using blow lamp which I use for silver soldering.

Second problem is the flux will corrode the steel if you do not clean off, some of the corrosion is within the seam if you do not get a good solid bead. However you could try electrical pre-fluxed solder as another option.

I gave up with soldering due to cleaning the flux and discolouring the steel, I did run a small bead of caulk then with a round tool made sure it went into the joints, I intend to seal as normal after that. All the rivet bolts I used thread lock on though, I built up the tender then went round all the rivets again with thread lock.

Have fun.

Richard.

At 2" scale the isoflex seals what gaps remain once you've pulled in the 9 million rivet bolts. I had to back off/tighten different sequences to get a fairly even pull and satisfactory butting up of the parts.