Slightly OT: Painting a helmet

No. You'll have to sand the helmet to remove any previous paint or primer, and to have a smooth surface, or your primer will not adhere.

Water-based acrylics will not stick to fiberglass. You'll have to use either a specific primer, or an acrylic lacquer primer.

Several thin coats rather than a thick one, and at least 24 hours of drying between coats. And don't get hit on the head when wearing that helmet...

be very carefull of your paints. some will render the pot useless.

Be very careful painting helmets particular "real ones" that are used to protect you head/brain. In most parts of the world stringent testing/certification processes exist and the simple application of a sticker (let alone paint) can render the helmet unsafe/illegal (some adhesives weaken the materials used in helments).

Be aware that few helmets consist solely of a fibreglass shell. Some cotain kevlars and various exotic polymer mixes.

If you want to paint a helmet identify the materials used in each section of it. Ask an experienced specialist painter (eg. high quality motorcycle helmet painter) for advice.

Primers will not damage fiberglass. It is pretty resistant to any paint-type chemicals. Go for it. You can even use lacquer and lacquer primers.