Can you paint your Tech Deck with acrylic paint?

It depends on what the helmet is made of, and the use for the helmet. Acrylic paint will stick to most non-oily surfaces, but will easily scrape off of slick and non-porous things. If the helmet is a football or motorcycle helmet, I would think that the paint would scrape off if it gets banged around. Also, acrylic paint softens when it gets wet, which makes it even more susceptible to being scraped off, so if the helmet is to be used in the rain, it would probably get wrecked. You can sand a slick surface to create a tooth on which the paint can stick but again, if it gets wet and softens, the paint will get damaged with only moderate banging around. Acrylic paint sticks quite well to rough, toothy or porous surfaces, but is best used for art objects that will be kept indoors.

You could find solvent-based paints (like model-paints) at a craft store like Michael's that may be more suitable for your helmet.