Fiberglass units are usually 'acrylic fiberglass' units. The acrylic is the shiny surface coating that is easy to clean and pretty and the fiberglass is the underlying structural support and strength. This is what most people buy. Look on the back side of a shower/tub unit and you will see the fiberglass strands.

If a unit were only acrylic (do they make them?), I would be concerned that the unit is going to be too flimsy and not strong enough.

In a bathroom remodel about 7 years ago I installed a shower stall by Sterling that was not acrylic-covered fiberglass. It was very easy to install and seemed sturdier than the acrylic-covered fiberglass units that I was also considering.
Also, if I recall correctly, it was only a little more expensive than the acrylic-covered fiberglass units (like $400 vs $300)

http://www.sterlingplumbing.com/solidvikrell.html

Lazy pup is correct although I think the retaining warmth argument on cast iron to be flawed. It gets cold (at least room temp) and holds the cold (especially if installed on an exterior wall in cold climate) and then the heat from your bathing water is transferred to the tub, only then does the cast iron warm and retain heat- after it has cooled your water off. So, in my opinion that's its downside when compared to acrylic or fiberglass.
Acrylics can be repaired although I don't know of any DIY kits. The difference between gelcoat fiberglass, AKA fiberglass, and acrylic is: gelcoat is a liquid plastic sprayed onto a mold, followed by layers of fiberglass reinforced with either wood, corrugated cardboard, foam or a combination of. Quality is determined by the thickness of gelcoat and the structural quality of the fiberglass and reinforcing materials. Look at the back and underside of the unit. There are high and low quality units.

Acrylics are a sheet of acrylic formed along with the same reinforcing materials as the gelcoat. Look for a thick sheet of acrylic reinforced by fiberglass layers that have been rolled on thicker and smoother than the gelcoat unit. Reinforcing materials tend to be better quality (more wood) Again examine the back, compare the weights. The acrylic has a thicker surface layer than the gelcoat and would be harder to damage or chip the unit all the way to the backing.

Cast iron is by far the easiest to clean and much less vulnerable to abuse by cleaning incorrectly. Never, ever use abrasives on acrylic or fiberglass, not even ones that claim to be safe for fiberglass, including Softscrub. Every time you use abrasive cleaners/sponges you scratch the surface which makes it harder to clean, you're making it easier for the soap scum to stick to it. The analogy would be prepping a surface for painting by lightly sanding it giving it 'tooth' for the paint (soap & scum) to bond to. A liquid cleaner with ammonia like Mr Clean works well. Waxing makes cleaning easier. Auto wax works so does Gel Gloss. Never heard of the furniture polish trick but it makes sense and would be easier to apply than the Gel Gloss/auto wax so you'd be more likely to apply as often as you should.

Last, lazypup is absolutely right about steel. Plumbing wholesalers won't even sell the stuff because 1/2 the load comes in damaged in transit and the rest gets damaged in the warehouse.

Wherever you are shopping, ask to see the back of the units. That tells the real story; you'll easily see the difference.

Thank you to all of you for some great information!!

Lazypup: You mentioned not to wax the floor or it will be slippery. Can I use the fiberglass rubbing compound on the floor? The tubs in my current house could use some help!!

I will go looking for the fiberglass repair kit for where the previous owner drilled holes in the side of the shower unit to install a soap dish right where I hit it with my elbow. Ouch.

If you are just filling a couple screw holes you may not need a fiberglass repair kit. If the backside of the enclosure is accessible you can cover the back of the holes with a piece of duct tape then get a "gelcoat repair kit" and fill the holes with gelcoat and let it set. Once the Gelcoat has set you can sand and buff it out for a final finish.

Perhaps if we consider the origins of fiberglass bathtubs it will help you understand how to care for one.

Shortly after World War II Brunswich Corp set out to modernize the appearance of their bowling alleys by replacing the old slatted wood seating with modern "Space Age" plastics in pastel colors. In order to do so they built an entire fiberglass production plant to manufacture the fiberglass seats and trim panels for their bowling alleys.

By the mid 50's they had pretty much revamped all their bowling alleys but they realized that they would need to keep the plant open to make repair parts for the future. Rather than downsize the plant they set out to find other products that they could make to keep the plant open. Formerly almost all private boats were made out of wood and boating was considered a rich mans hobby. Some marketing genious got the idea of making fiberglass boats for a fraction of the cost of a custom wood boat, thus was the origin of "Bayliner Boats" which went on to become one of the largest builders of private watercraft in America. From that origin someone got the idea that if fiberglass could keep water out of a boat, it could also keep water in a tub or shower, thus they began making tubs and showers as well.

In the early 50's General Motors seized upon fiberglass as the material of choice for their "Corvette" bodies and today nearly all automobiles have numerous fiberglass body components.

Now, whether they were making molded contour seats for a bowling alley, a boat hull, a bathtub or a classic car body the methods of constucting the fiberglass remained the same.

Some years ago I was touring a fiberglass boat building plant and the guy directing the tour told us that the very first step of making a fiberglass boat is to install the pinstripping and paint. Baloney I thought, how can you paint a product that does not yet exist? Was I in for a surprize, the guy was absolutely correct. Step one is painting.

They begin with a mold of the finished product. First they place the pinstripping and decals on the walls of the mold, then they spray in the gelcoat. After the gelcoat sets they build up layers of fiberglass cloth impregnated with resin, followed by fiberglass matting or in some instances they blow in chopped fiberglass and resin until they get the desired thickness of buildup. The mold is then set aside to allow the resin to cure and when it is all hardened they attach a high pressure air line to a nozzle on the bottom of the mold, turn the air on and voila' out pops a finished product.

Understanding that a fiberglass tub or shower is made the same way as a fiberglass boat hull or autobody it made perfect sense. If a fiberglass boat or autobody neglected for any length of time it soon begins to look dry and faded but with a little rubbing compound and elbow grease we can restore the lustre. Why not try that on a tub? Sure enough, it worked fine. In fact, I once had a tub that had been neglected for a couple years and it had a dirty dingy soap scum that defied all efforts to clean it. I thought to myself, whats to loose here. In a worst case scenario I will have to change it out so for the moment why not try an experiment. I went out to the garage and got my trusty can of boat hull "bottom cleaner". The same soap based product that I used to clean that nasty brown river scum off the bottom of the boat and sure enough, just as it cleans the river scum off the boat, it cleaned the bathtub rings out of the tub. Whats more, it cleaned the gooves in the non-slip texture in the bottom of the tub, fantastic I thought.

I then grabbed my low speed autobody buffer with a wool pad and some fiberglass rubbing compound and polished out the entire tub and shower enclosure in the same manner as polising the boat or car. (The key is to work slow with low pressure on the wool pad. Think of this as if you were polishing a classic corvette for a car show).

After its buffed out with the rubbing compound apply a light coat of fiberglass wax to the entire surface. This will protect it from oxidizing.

For day to day maintenance we rinse and clean the tub in the usual manner then apply the light film of Johnsons Pledge furniture polish to all surfaces except the bottom walk area of the tub.

My brother-in-law took my fiberglass reasoning to the next level. When he moved into his house he was on a shoestring budget and they had an ugly 60's pink tub & enclosure. He went to the marina and bought a quart of fiberglass "Topside" paint and painted his tub, and too my amazement it looks very good.

A word of caution that he got from the marine dealer. If you r going to paint a tub be sure to use "Topside" paint, not Hull or bottom paint because many of the marine bottom paints have toxic chemicals that are designed to leach out of the paint to retard the growth of algae or barnacles.

Lazypup, you are great!

I don't have access to the back side, but the reason there are screw holes is that there are wall anchors stuck in there. I could cut them below the surface and cover them with the gelcoat repair kit. . . is that only for really thin repairs or something? What is the difference between gelcoat repair kits and fiberglass repair kits? How big a hole could I plug with either?

Hmmm. . . I see it comes as a resin and a paste. Rats. I have one really thin spiderweb of hairline cracks at the bottom and one gouge from the screw holes. I probably need both kits!!

Does anyone make them specifically for the color of bathtubs- in other words, can I get Harvest Gold premixed so I don't have to worry about how to mix it?

Thanks!!!

Lazypup, please come back!!
Is there anyway to find a pre-tinted repair kit?
Thanks again for the great info above!

Dizzy,

You stated that you have a number of fine hairline cracks and a screw hole.

You should not need a fiberglass repair kit.

Your tub has a structural base of fiberglass but the GELCOAT finish is about 1/16" to 1/8" thick. For the hairline cracks you may be able to wet sand them out using very very fine emery cloth then buff out and wax for a final finish. (There is a small supply of the emery cloth supplied in the GELCOAT repair kits)

I am not aware of any companies that supply a premixed color . The problem is that GELCOAT like all paint or pigmented surfaces does fade with age so it would be nearly impossible to premix an exact color match.

The good news is that when GELCOAT dries it retains the same color so if you mix a fairly good match it will be fine. Keep in mind that your are only filling a small screw hole. Once it is filled, sanded, buffed out and waxed it is highly unlikely that anyone would ever notice it unless you point it out.

Thank you for setting me straight on terminology! Sounds like the gelcoat repair is what I need. I appreciate your help!

Hey!!!! Guess what? Someone use a gelcoat repair kit on my bathtub!!! I started cleaning it and noticed this spot that is just a little off. Never noticed it in the ten years we have lived here.