“I am certain other readers of your blog would be interested in you expounding a little more on where you consider the break to occur between abandon and repair, and the salient factors you would consider.”
Probably the best way for me to answer this question is to explain what is involved in repairing a lugged steel frame, and how I would approach it.
The easiest tubes to replace are the top and down tubes. I would cut the damaged tube with a hacksaw a few inches from the lug. Heat the lug uniformly, and to do this I would often use two oxy-acetylene torches, one in each hand on either side of the lug.
With one torch heating first one side then the other, one side will cool as you move to the opposite side. If you don’t have the luxury of two torches, then a simple hearth made out of fire bricks, to hold the heat, is called for.
[Left: Brazing Hearth. Picture from Mercian Cycles.]
If you don’t heat the joint uniformly you risk breaking the lug. With everything at uniform heat, (Orange red for brass, dark red for silver.) you can simply grab the short piece of tube with pliers, twist, and it will slide right out.
Some framebuilders pin the lugs to ensure they don’t move during brazing, so you need to look to see if there is a pin sticking through inside the tube. If there is, reach in and grab it with a pair of needle nose pliers, heat the spot on the outside where the pin is, and pull the pin through inside to remove. Then you can go ahead and remove the piece of damaged tube.
After the frame has cooled, the inside of the lug can be cleaned up using a carbide burr in a hand held grinder. With a tube removed the frame is very flexible and will easily spring apart to allow the replacement tube to be inserted.
The most difficult tube to replace is the seat tube. It is a simple matter to hacksaw the damaged tube out and remove the lower piece from the bottom bracket. However, removing the piece of tube from the seat lug is a completely different story.
To heat this area uniformly you are going to melt the joint where the seatstays are attached to the seat lug, and in all probability, the seat lug will move or come off the top tube.
Some repair shops have a set up where they can machine out the piece of seat tube using a reamer or correct size milling cutter. This way no heat is used in the removal of the old tube, but you still need to exercise caution in re-brazing the joint. Care is needed so you don’t melt other parts of the joint.
The way I approached this repair, was to replace the seat tube complete with a new seat lug. This way I could cut out the damaged tube, and hacksaw the seat lug on either side of each seatstay.
Next I would heat and detach the rear brake bridge on one side only, and spread the seatstays apart by inserting a six inch piece of wood between them. Then the cut pieces of seat lug still attached to the top of each seat stay could be removed by heating from the inside without melting the entire seat stay cap.
The final piece of seat lug had to be removed from the top tube by heating uniformly and then pulling it off with pliers. After cooling and clean-up, it is a relatively easy matter to put the new seat tube in place, along with the new seat lug, and re-braze everything in the normal way.
It is rare to have to replace a chainstay or a seatstay, but these would be cut out with a hacksaw and the remaining pieces removed by heat. It is often easier to replace a both seat stays, even if only one is damaged, rather than try to match one new one to an existing seatstay.
Slightly bent seatstays can be safely straightened, (Providing they are not kinked.) as can front fork blades. These are much thicker that the main frame tubes.
I would only repair my own frames, and did this as a service rather than as a money making proposition; most other framebuilders are the same. A big consideration is not so much, can a frame be repaired, but can you find a reputable framebuilder to do the work, and what will be the cost?
I fail to see where buying a damaged frame on eBay or anywhere else is a worthwhile proposition. With used steel frames still plentiful and at reasonable prices, in most cases the only reason to repair a frame would be one of extreme sentimental value.
The frame pictured at the top is my own Fuso frame damaged in my accident last December. The frame cost me $260, and if I can find another for close to that price it would clearly be less than the cost of a repair and re-paint. On the other hand, it may be difficult to find another in this size (52 cm.) so I may have to consider having it repaired.