trailer repair, just weld in a new bit ?

[penfold]

A scarf joint helps avoid a weak point, although practically fishplates are adequate unless the trailer is highly stressed or badly designed.

 

[DownWest]

A scarf joint helps avoid a weak point, although practically fishplates are adequate unless the trailer is highly stressed or badly designed.

Quite, though the trailer in the photo is quite well put together to cater with loads.

 

[Daydream believer]

The OP would be better welding a plate over as I suggested, rather than IN, because cutting a piece out & putting a new section in may involve butt joints & a good fit. The weld would be more difficult. By having a plate placed over the face, the weld would run along the internal angle formed by the plate edge & the main frame. A few pigeon droppings will not be a deal breaker. Any fumes on the inside of the box will not affect the welder & those in the area of the outside are actually quite minimal & forumites are making a big deal of a small matter. Provided the operative takes care not to inhale the fumes. Do the work outside & keep ones face well clear is a not difficult

 

[Caraway]

Quite agree. The trailer is not to be travelling at 60 mph on motorways. More like 3 mph in a boatyard.
Just plate it over, long welds on good metal away from the rot. It is not going to fail.
Do just as much as needed. No more.