In-situ Buttering is deposition of a layer of weld metal on the susceptible surface (‘buttering’) prior to filling the joint ensures that strains due to the weld passes in contact with the steel are low.
Or it is special buttering type different from the others – See Buttering vs Buildup in welding
The deposition of low strength weld metal is deposited firstly on the susceptible plate before filling the joint with another electrode. However, before adopting this technique, design calculations should be carried out to ensure that the overall weld strength would be acceptable.
Some O&G project has allowed using of In-situ buttering for exceed fit-up gap spacing after big-block erection. There’s no chance or high cost, or danger of falling to remove the structure. In this case, the In-situ Buttering is considering as Buildup welding because only one type of electrode is used for closing the gap.