Can interior flat paint be used on the exterior as a primer coat, under exterior paint?

No it won't bond properly and will peel away. It will take a lot of effort and then the top coat will be wasted,Can interior flat paint be used on the exterior as a primer coat, under exterior paint?
Primer usually contains additives that reduce mildew and seal a surface so that it can accept paint.





The only other bit of info I can share is that I painted the exterior of my house with ';recycled paint'; which is a product that blends together a gazillion blobs of left-over paint that people wish to dispose of. This would include interior and exterior paint. The contractor who did the job was very reluctant to use the product, but in the end, he said it had gone on really well and he would use it again. The paint job has lasted for 5 years so far in a very rainy climate.Can interior flat paint be used on the exterior as a primer coat, under exterior paint?
No. It will not bond with the paint all ready there, and will peel off really quickly.
This would be very risky. The aforementioned ';recycled paint'; may not have interior and exterior paints mixed together.