First, in general the cause yellowing:
1. 1K white spray, not waiting to dry completely, directly spray varnish, easy to turn yellow.
2. paint film absorbs moisture, direct light cover, also easy to turn yellow, especially in rainy weather
3. the construction of environmental humidity is too large, it is easy to turn yellow, white, and the air contains water vapor to pay up the film
4. last night when painting equipment not clean cause.
5. varnish expired, the curing agent is too poor, not yellowing.

The reason to exclude
In retrospect, then spraying 1K white sure is completely dry before spraying varnish, years of experience to do. This paint use not long time, it did not happen, but look at the production date is in production, can also very new, so this is unlikely. In addition each use spraying equipment are clean is one of my habits, also may be the reason.
The only possibility is that there is water vapor, ah, this only see light suddenly.
These days the weather has been overcast overcast yesterday rain, spray room humidity is too high, the moisture penetration into the film. Now only grinding again, so we advise people not to paint in the rain, because the humidity is too large, the film is prone to problems.
Also note:
If it is sprayed 2K plain white, then varnish will produce yellowing phenomenon, 2K itself is plain white yellow phase, then cover the light, yellow mother will emerge in the layers of varnish.