Mix the skim coat with cement if you want it last long. Else the skim coat will fall off during rain

I hope you are not thinking of using the white stopping compound anywhere near the exterior. No matter mix with cement or not. Some half-past-six contractor / foreign worker told you can use?

Please use something like SikaWall^® 198 that is meant for external use. https://mys.sika.com/en/construction/plaste...nish2-in-1.html

To be honest, skim coat is not an issue. Workmanship is the key.

I paid my contractor to do inside my house. After switching on my lights after dark, I was glad that I did not waste my money to do the external wall.

Some video clips show how the contractors in China do it nicely. But, honestly, few contractors in Malaysia have those equipment and I doubt their foreign workers could deliver the same effects.

I did explore various options available to do smooth external walls. Installing facade cladding probably is better. But, this is costly.