Any extra effort you try to put afterwards doesn't have any effect and it wont change that first impression.
Is this true? I think it is, and I'll tell you why.
Despite what people believe, first impression we get from someone or something is not related to what we think of it, since reasoning comes from an analysis our brain performs with the given information. And even though it can happen really fast, that reasoning is not what defines what we feel about that person, or object.
There is a step that goes before the reasoning, a step where all the collected information doesn't have any importance. That step is called Instinct. That sixth sense we all have, which my father use to repeat to us: "it never fails, it never gets it wrong".
Think about the following scenario:
In a job interview with a possible candidate, he comes very well dressed, clean, perfect hair, that tells your brain he cares. He pulls out a rather impressive portfolio of previous work, that tells you he can. He speaks good english, carefully selected words. That tells you he knows. But somehow, you just can't get yourself to trust him. Something about slight feeling of fakeness that rang a bell at the first handshake.
Instinct is that fragment of a second that tells you everything with a feeling rather than words. That sense of knowing that the candidate wasn't the right person for the job at the first handshake. That little bell we mute with explanations, information to later on realize it was in the right and that the brain got it all wrong, again.
Instinct doesn't require any information, it's always there with the right choice, even when we don't listen to it.
The problem with reasoning is that it needs external (not reliable) information which can be manipulated, false, or incomplete. We also tend to mix other things that may or may not have relation with the situation.
At the end, all I can say is that we need to make sure we show what we really are, because we can't give any better impression than the real one.