Iivi Anna Masso has written a pretty good article about the similarities between the attitudes of the most avid pro-immigration and anti-immigration people.
One of the things she said was that in the eyes of the nationalists immigration is in itself a bad thing, and immigrants are bad people.
The nationalists naturally rushed to deny it, but she does have a point.
First of all, there are nationalists who really are against all immigrants. Some are ethnic supremacists. Some are just neophobes. Some say things like "but of course I am not against all immigrants, if people are obeying the law and working for a living they are welcome", but somehow tend to find arguments against people who are coming here to work if the conversation is specifically about them.
Those are a minority of nationalists. But I think quite a lot of the public image problem lies with the majority. In my experience, which in mostly based on the Internet forums, an average Finnish nationalist:
a) tends to believe that most immigrants come from Africa and Muslim countries, don't integrate, and don't work,
b) knows that there are people who come from the Western countries, and that some immigrants, both from the West and the third world, who do in fact integrate and work,
c) really has nothing against well-integrated immigrants,
d) uses the word "immigrants" in conversation to mean badly-integrated third-world immigrants.
See the problem? If you read some forum regularly, you have no problem figuring out who really means what. If you challenge people on something they say, they explain themselves better, and sometimes they are lying, but usually they aren't. But to an outside observer who is neither familiar with the participants, nor ready to ask extra questions, the conversation sometimes does look bad, and for a reason.
The problem is not limited to nationalists: pretty much all political forums, and many non-political ones, have a lot of people making strong statements they don't necessarily mean, letting off steam, joking in a way not likely to be understood by the outsiders, and in general behaving like they are "among their own" and will therefore be understood. It's sort of easier and more fun that way, but one should hardly be surprised at being misunderstood.