My hubby repainted our fiberglass "bathtub" in our 63 Bambi and he used a spray can of the Krylon Plastic paint available at about any hardware store. It hasn't been too long yet (a few months) so I cannot attest to how it will hold up. However, so far, so good. He said I shouldn't use cleanser or anything on it, just mild soapy water and a rinse.
Good luck! You can also use the search box above on the blue toolbar and key in bathtub paint ~ you will get quite a few hits.
My husband also used a Krylon Plastic paint on ours. It looks really nice. We did all the counter space in there. He ran some water in it and it held up pretty good. I would think since its made for the outdoors it should work nicely. Picked it up at lowes, walmart carries it to. Very inexpensive. heres some pics.
Are you sure the tub in your '79 is fiberglass? It may be ABS plastic.
I just did my tub using a product that claims its good for fiberglass. porclein and plastics. It was also used on the Classic Rides restoration.
The product is called Tubby from www.tubbyusa.com. It's a two part epoxy paint and comes in a kit with everything you need and very detailed instructions. It is brushed and rolled on which is included in the kit.
I don't know how its going to hold in up in the long run but it seems pretty nice right now.
i used "elastomeric" paint made for platis vinyl and leather re-finishing primarily automobile seats and upholstery. it stinks going on but does not come off. period. i also used it on all of the plastic end caps and window mouldings, etc..
i used a marine paint manufactured by interlux --- it is fiberglass based and one part. it came out great but was easy to "run". it is pricy at $32.00 per quart.... my unit is a rear bath 1978 sovereign and had been broken in the shower corner which i re-inforced with fiberglass mat and resin....