Actually I think that "The paint might be washed off if you didn't wait long enough." could be absolutely fine depending on the context. Imagine this:
"I didn't leave very long between coats and it's not drying very fast, and it's about to rain - do you think there will be a problem?"
"The paint might be washed off if you didn't wait long enough."
OK, it's quite a specific context, but we can't really write the sentence off as totally incorrect.