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I have also had success using hair spray to initially seal the chalk with. I use the pump bottle kind that I can spray above the model, letting it mist down onto the model's surface. I have very little loss of "color" from the chalk.
I use Valspar Clear Flat for sealing mine. I don't spray it spray it directly on but stand back and spray above the model and let the mist fall down on it.
The cheapest brand you can find. After misting it on, by spraying above the model and letting it fall down onto the surface, just like Jeffrey said, after it dries, then spraying on a dull coat.