Castano shot a 10-under-par 61, playing his remaining nine holes at Sentosa Golf Club in 30 to lead on 15-under-par 127.
"I was lucky because yesterday I didn't tee off before the thunder storm. I took my time. I took it easy and started well with three birdies. It was great and then I had a good start again this morning. To play well you need to hole putts and that is what I did," Castano said.
"I found something with my swing. I have been injured for six months at the beginning of the year and it took me a while to find my swing and what I want to do with the ball. I finally did and of course the big thing was the putting.
"I changed my putting method at the Czech Open in mid August . Since then my putting has improved a lot. It's the claw grip. I am just imitating Sergio Garcia to see if I can win as many tournaments as he does."
Castano is a four-time winner on the European Tour and he was clearly delighted to be leading the US$6million championship in Singapore with two rounds to go.
"There is no better place to be," he said.
Italian Edoardo Molinari signed off with a 68 on Saturday morning to join James Morrison of England on 130 for joint-second spot at 12-under-par.
The third round will be completed on Saturday weather permitting.