In good order, correct

"A dealer said that this is not a Tiffany lamp because the base finish is wrong" "I'm an antique expert and let me tell you that the finish is as right as rain. This is a perfect Tiffany lamp from 1904 and is worth over $ 100,000"

"Un commerciante mi ha detto che la mia non è una lampada Tiffany perché la finitura della base non è corretta" "Io sono un antiquario esperto e ti dico che la finitura è corretta e perfetta. Questa è una bellissima lampada Tiffany del 1904 e vale più di 100.000 dollari"

Right as rain follows a long list of curious similes. It was first recorded at the very end of the nineteenth century, although the allusion to rain is unclear. It seems more just a play on words, with a very good alliteration, which made it quite popular