A quick search on Google shows that EvapoRust is available in Singapore. I would plug/tape the holes in one section (rear triangle, top tube-down tube, seat tube) of the frame at a time and fill those tubes with EvapoRust. Let it sit for an hour or so then drain it out into a bucket to reuse (after straining/filtering or letting it settle out) in another section until all the tubes had been derusted. Could probably seal every hole from the head tube back, stand the frame up on the rear dropouts and fill the whole thing at once. The metal should be degreased before derusting with EvapoRust.