# Fiberglass tub repair: Our hobby epoxies vs. "commercial" repair kit resins

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Ok, here's an oddball question. One of the fiberglass tub/shower inserts in our home has developed a 6-7 inch long crack in the bottom and, having done some rocketry related fiberglassing once or twice, I feel 100% qualified to attempt a half-ass repair myself in hopes of avoiding spending lots of $to replace the thing. As such, I'm hoping somebody here has had some experience with similar fiberglass repairs. A handyman guy that we've used (a.k.a. - my brother in law) suggested just getting a fiberglass repair kit and using that. But in looking at those online, I'm left with a feeling that the "resins" these kits come with are probably not as strong as my trusty Aeropoxy or USC 150 brands. Should I substitute some of my own epoxy? Or are the properties/usages different enough that the kit resins are better for this application? Anyone here have any experience with these repair kits at all? #### blackbrandt ##### That Darn College Student From my limited knowledge... Those tubs are probably made with polyester resin. You're going to be using epoxy resin. The two are not compatible, and will not stick to each other. You would need some polyester resin (which you can get from lowes or something) in order to have it bond. Please feel free to correct me if this is wrong. #### PropellantHead ##### Well-Known Member From my limited knowledge... Those tubs are probably made with polyester resin. You're going to be using epoxy resin. The two are not compatible, and will not stick to each other. You would need some polyester resin (which you can get from lowes or something) in order to have it bond. Please feel free to correct me if this is wrong. Hmm... you might very well be right. I have only just enough knowledge to get myself into real trouble so this definitely bears some investigation. I'll research what I can about this particular insert and see if this is polyester we're dealing with. Thanks for the reply! #### Cl(VII) ##### Chris Bender, Lab Rat TRF Sponsor Perhaps you can find the msds on one of the commercial repair kits to find out what class of adhesive they employ. #### rharshberger ##### Well-Known Member Actually BlackBrandt is nearly dead on the resins used in bathroom showers and tubs is a MEK-P catalyzed polyester resin, with a gel-coat (another polyester resin with color) surface, the molds are sprayed with gel-coat and after short while then sprayed with a chopper gun using fiberglass roving to create the FG mat (and uneven appearing back of tub). As for the compatibility of the two resins (epoxy vs polyester) I don't have a clue. My father for 10 or so years owned a fiberglass and cultured marble shop where they made custom bathroom sinks, tubs and showers, repairs and touch ups if necessary were always done with the same product they were originally made from. A commercial FG repair kit is probably the best way to go. #### Steve Shannon ##### Well-Known Member TRF Supporter Ok, here's an oddball question. One of the fiberglass tub/shower inserts in our home has developed a 6-7 inch long crack in the bottom and, having done some rocketry related fiberglassing once or twice, I feel 100% qualified to attempt a half-ass repair myself in hopes of avoiding spending lots of $ to replace the thing. As such, I'm hoping somebody here has had some experience with similar fiberglass repairs.

A handyman guy that we've used (a.k.a. - my brother in law) suggested just getting a fiberglass repair kit and using that. But in looking at those online, I'm left with a feeling that the "resins" these kits come with are probably not as strong as my trusty Aeropoxy or USC 150 brands.

Should I substitute some of my own epoxy? Or are the properties/usages different enough that the kit resins are better for this application? Anyone here have any experience with these repair kits at all?
Before you fix it, consider whether a repair will be creating an enclosed wet environment for rot and mold to grow. Also, why did it crack?
I had a chip fixed in a fiberglass shower enclosure. It was definitely polyester, not epoxy and the guy who did it did a great job of matching the color, but my surround was not cracked all the way through. 20 years later I can't see where he made the repair.

Steve Shannon

#### cbrarick

##### Wildman CT
maybe i'm stupid.....but aren't our premium epoxies more expensive then the repair kit, ounce for ounce?
Just saying.... stay with the cheap home depot kit and keep the good stuff for your rockets.

#### markkoelsch

##### Well-Known Member
Actually BlackBrandt is nearly dead on the resins used in bathroom showers and tubs is a MEK-P catalyzed polyester resin, with a gel-coat (another polyester resin with color) surface, the molds are sprayed with gel-coat and after short while then sprayed with a chopper gun using fiberglass roving to create the FG mat (and uneven appearing back of tub). As for the compatibility of the two resins (epoxy vs polyester) I don't have a clue. My father for 10 or so years owned a fiberglass and cultured marble shop where they made custom bathroom sinks, tubs and showers, repairs and touch ups if necessary were always done with the same product they were originally made from. A commercial FG repair kit is probably the best way to go.
+1 to this. Been there, done that.

#### Woody's Workshop

##### Well-Known Member
TRF Supporter
If you don't care color matching your tube, and want to do it yourself, Pm me and we can talk.
I have years of experience working on corvettes and boats and other fiberglass panels.
Once a resin cures, it doesn't make any difference what product you use.
The key thing is what you have to bond to, and the appropriate product to use for that.

#### new2hpr

##### Well-Known Member
Woody's right. Once cured, the resin type doesn't matter. You're doing a purely mechanical bond at this point. Hopefully you've got access to the back side of the repair spot, so you can bond to the "hairy" chopper gun fiberglass to get a good mechanical grab. On the interior, once you've got a structural repair underneath, I'd try the thinnest runniest epoxy you can find, so you can squeegee it into the crack. West Systems and a heat gun maybe? I'm sure Woody has tons of experience to tap after working Vettes and boats... Let him guide you.
-Ken

#### T-Rex

##### Ordinary Average Guy
TRF Supporter
One thing that I will say here since it has not been mentioned. If you decide to go with polyester resin (i.e. Bondo) and don't know, make sure you have PLENTY of ventilation. The poly resins have some serious fumes until cured.

##### Well-Known Member
If you don't care color matching your tube, and want to do it yourself, Pm me and we can talk.
I have years of experience working on corvettes and boats and other fiberglass panels.
Once a resin cures, it doesn't make any difference what product you use.
The key thing is what you have to bond to, and the appropriate product to use for that.
That was kind of my thought as well. Nothing I've seen indicates that these repair kits form any kind of chemical bond so as long as I do great prep on the surface, I *should* be ok. I'll PM you later this evening when I have more time to linger at the keyboard.

##### Well-Known Member
Woody's right. Once cured, the resin type doesn't matter. You're doing a purely mechanical bond at this point. Hopefully you've got access to the back side of the repair spot, so you can bond to the "hairy" chopper gun fiberglass to get a good mechanical grab. On the interior, once you've got a structural repair underneath, I'd try the thinnest runniest epoxy you can find, so you can squeegee it into the crack. West Systems and a heat gun maybe? I'm sure Woody has tons of experience to tap after working Vettes and boats... Let him guide you.
-Ken
Unfortunately, I do not have access to the back side of the insert. This will be a top surface repair. We were planning to remodel this bathroom in 3-4 more years anyway so I don't mind if it's an ugly repair so long as it stays leak free until we rip it all out by the roots.

##### Well-Known Member
Before you fix it, consider whether a repair will be creating an enclosed wet environment for rot and mold to grow. Also, why did it crack?
I had a chip fixed in a fiberglass shower enclosure. It was definitely polyester, not epoxy and the guy who did it did a great job of matching the color, but my surround was not cracked all the way through. 20 years later I can't see where he made the repair.
Rot shouldn't be a problem as it's a very recent crack and has leaked 2-3 times. As to why it cracked, well it's probably just physics. It's a 30+ year old insert and I'm 285 pounds. Something was bound to give eventually. As I mentioned in another msg, I don't care if it's an ugly repair since this thing is coming out in a few years anyway as part of a big remodel we have planned.

#### Marc_G

##### Well-Known Member
I suggest a somewhat different approach, having dealt with this general kind of thing a number of times (never on a tub though). I understand there's a ~7" crack at the bottom of the tub, right? And no access to the underneath part?

I suggest doing this:
cut out a section around the crack, say 9 inches long by a couple inches wide. Do this neatly, as you will save and reuse the part you cut out. You now have a biggish hole in the tub, and access to the underside through it. Create a patch affixed to the underside of the tub, much wider than the hole, say two inches all around the hole. This can be a bit finicky with the limited access but you'll engineer a way, probably with the patch on a more or less rigid surface like cardboard underneath. Pull the patch up and into place in epoxy or whatever you have slathered on the underside of the tub. Allow to fully cure, making sure that the patch doesn't interfere with placement of the piece you cut out. Once cured, it's now waterproof etc, but looks horrible of course. Now, epoxy (or whatever) in the piece you removed, color-matched as close as you can to the tub color. You'll only see the kerf lines around the hole, and maybe a bit in the middle where the crack is presuming you fill it in.

When all cured, you now have a very resilient patch from the underside that should hold up. IF you are super paranoid you can put another layer over on the inside of the tub.

#### Steve Shannon

##### Well-Known Member
TRF Supporter
I suggest a somewhat different approach, having dealt with this general kind of thing a number of times (never on a tub though). I understand there's a ~7" crack at the bottom of the tub, right? And no access to the underneath part?

I suggest doing this:
cut out a section around the crack, say 9 inches long by a couple inches wide. Do this neatly, as you will save and reuse the part you cut out. You now have a biggish hole in the tub, and access to the underside through it. Create a patch affixed to the underside of the tub, much wider than the hole, say two inches all around the hole. This can be a bit finicky with the limited access but you'll engineer a way, probably with the patch on a more or less rigid surface like cardboard underneath. Pull the patch up and into place in epoxy or whatever you have slathered on the underside of the tub. Allow to fully cure, making sure that the patch doesn't interfere with placement of the piece you cut out. Once cured, it's now waterproof etc, but looks horrible of course. Now, epoxy (or whatever) in the piece you removed, color-matched as close as you can to the tub color. You'll only see the kerf lines around the hole, and maybe a bit in the middle where the crack is presuming you fill it in.

When all cured, you now have a very resilient patch from the underside that should hold up. IF you are super paranoid you can put another layer over on the inside of the tub.
I would just drill a hole, inject spray foam, and then patch the top.

#### rharshberger

##### Well-Known Member
I would just drill a hole, inject spray foam, and then patch the top.
Better yet figure out how to inject the high density two part foams similar to what we use in rocketry, then plug the holes, the foam will provide more support for the bottom of the shower. When the remodel is done make sure and set the shower unit on a bed of mortar if possible then the bottom will be supported and not crack later.

#### hornet driver

##### Well-Known Member
I'm not as optimistic as others about a short term repair. I assume the crack is all the way through and not just in the gelcoat. If that is the case,you have to get underneath to reinforce the area so it does not flex. A surface repairs life expectancy will be measured in weeks not months or years. Flexing from standing on or near it, heat cycling, and water intrusion are your enemies and they all happen at the same time. Let us know --down the road -- if your repair works and lasts for any length of time. I'd be interested in what you did. Just thinking out loud--a large surface repair might do the trick since it spreads the loads but you adhesion and seal would have to 100%. I'd go ahead and put in a new insert, but what the hell, give it a try---H

#### TopRamen

##### SA-5
Through the use of pinning and rivets, not to mention drill through bores to allow for doweling, you can have maximum strength with whatever material you wish to use. Jointing can become secondary and unique to each joint.

#### Woody's Workshop

##### Well-Known Member
TRF Supporter
First of all, the crack is a separation of material causing expansion.
Therefore it is under stress, and you need to release the stress first.
To do this, determine the farthest points at each end of the crack, and drill a 1/4" hole.
If you can see separation in the hole opposite the crack, you need to drill farther out.
Using a cut off wheel of around 1/8", cut all the way through at the top surface of the crack, stopping at the holes at each end.
Stress is now released. As best as you can, V cut at 45 degree angle to the bottom of your cut with the cut off wheel.
Tape around your cut and holes about 1/4" away with duct tap, clean the area with lacquer thinner and sand the gel coat surface so it's all ruffed up.
Remove the duct tape, clean with thinner again, and re-tape with painters tape or masking tape.
Cut a piece of thin cardboard or card stock long enough to leave 2" longer and wider than the cut. Lay it over your cut, center it, and punch holes at the center of your drill wholes.
Now comes the fun part. Use string, thread a piece through each whole in the card board and knot the back side with one knot. To be save, use pieces of 24" or more.
Feed it through the cut sideways, when fully inserted, turn it and fit the string through your drilled holes.
Leave the card board hang low by weighting the string with something and the best you can coat as must of the card board as possible as thick as will stay on the card board.
Pull it up by the strings snug against the bottom of the tub and re-weight the string down so it doesn't slip.
While the underside is still uncured, fill the crack and wholes. (I would assume you will be using 15 minute epoxy here, I would)
As the epoxy sets up so its firm, but still slightly pliable and won't sag under the tub, pull the strings out, knots and all, and dab in some epoxy.
Using a bondo spreader, or anything with a straigt edge, smooth out level with the tape. Remove the tape and smooth the edges around your sanded area.
Wait a good 12 hours to use the tube.
While the crack actually added a flex point in the tube, the epoxy from the bottom, through the cut and on the top will be flexible enough to give a little.
The wholes drilled at the ends of the crack relieved the stress so it doesn't migrate any farther when flexing.
Feel free to add pigment to the epoxy to try and match the tub color.
This is what I would do. And it's the best you can do without access to the bottom side of the tub.
Try not to exceed the width of the crack as your drill wholes. If that's not possible, use bigger wholes.
If you decide to cut a patch out as suggested above, (I DO NOT recommend this) make sure you drill a whole in all 4 corners FIRST!
Good Luck!

#### Screaminhelo

I suggest a somewhat different approach, having dealt with this general kind of thing a number of times (never on a tub though). I understand there's a ~7" crack at the bottom of the tub, right? And no access to the underneath part?

I suggest doing this:
cut out a section around the crack, say 9 inches long by a couple inches wide. Do this neatly, as you will save and reuse the part you cut out. You now have a biggish hole in the tub, and access to the underside through it. Create a patch affixed to the underside of the tub, much wider than the hole, say two inches all around the hole. This can be a bit finicky with the limited access but you'll engineer a way, probably with the patch on a more or less rigid surface like cardboard underneath. Pull the patch up and into place in epoxy or whatever you have slathered on the underside of the tub. Allow to fully cure, making sure that the patch doesn't interfere with placement of the piece you cut out. Once cured, it's now waterproof etc, but looks horrible of course. Now, epoxy (or whatever) in the piece you removed, color-matched as close as you can to the tub color. You'll only see the kerf lines around the hole, and maybe a bit in the middle where the crack is presuming you fill it in.

When all cured, you now have a very resilient patch from the underside that should hold up. IF you are super paranoid you can put another layer over on the inside of the tub.
I am going with Marc's suggestion for strength. This is much like a repair on aviation composite panels where there is no backside access. Thing is though, the repair gets its strength from overlapping layers on the outer surface. I agree with Hornet Driver, repeated flexing is going to be brutal on this repair. If you want this to have a chance survive 3-4 years, I feel as though you're going to need to layer glass on the top surface to spread the load.

#### Marc_G

##### Well-Known Member

Any repair like we described will be uncomfortable to sit on when you bathe.

How about this: buy a smooth sheet of flexible something or other. Like a smooth bath mat for example. As wide as the tub bottom more or less, say two feet long. Spray the underside with the permanent type of contact adhesive, and a similar but slightly smaller area on the tub (mask surrounding area. Let sprayed adhesive dry to extent instructions state. Apply mat over the intended area and smooth out.

Let the adhesive dry/outgass/cure or whatever it does. You now have a flexible repair, with square feet of grip area, the crack being centered in the middle. And being a bathmat or similar, it's comfy to sit on. This will last a few years and have zero chance of a sudden catastrophic failure.

Marc

#### Screaminhelo

I would be a bit concerned about structural integrity of the tub if you used the bath tub mat alone. I admit that effecting a long lasting repair may be a bit lumpy if you are going for cheap and quick but you could put the tub mat over the repair.

#### jd2cylman

##### Still not Carl... ;-)
TRF Supporter
This is the exact reason why I bought a steel tub with an insulated backing. My brother's fiberglass tub in his bathroom cracked, because the builders didn't properly build the mortar base under the tub to support it properly. So, when I remodeled my bathroom, the lesson was already learned. On the plus side for me, the steel tub was actually cheaper. The other thing with fiberglass tubs is that some have antibacterial crap infused into the surface. This (some people believe) leads to more resistant strains of bacteria... My steel tub was just plain old porcelain over steel. My \$.02

#### Woody's Workshop

##### Well-Known Member
TRF Supporter
Personally, I prefer steel tubes as well.
But in the winter time, you have to fill them with pure hot water as it takes much more to heat the tub too for a bath.
But I shower, I could never see sitting in dirty water a reliable way to get clean.