Fiberglass Resin Recommendations?

[David Tilston]

Please explain the "extra strength" of epoxy. It is my understanding that the strength of fibreglass is mostly in the glass. What are the benefits of epoxy?

[MCDenny]

Originally Posted by D White

And you must use an epoxy primer because nothing sticks to cured epoxy except epoxy, .

Absolutely untrue. I've built 20 boats with epoxy used as both glue and coating. Both latex and oil based paint stick just fine. No primer at all is needed. The epoxy needs to be sanded first, just as any glossy surface would.

[MCDenny]

Originally Posted by David Tilston

Please explain the "extra strength" of epoxy. It is my understanding that the strength of fibreglass is mostly in the glass. What are the benefits of epoxy?

Excerpts from
Resin Properties for Composite Materials

Epoxy sticks to stuff better:

"Of the three resin types discussed in the article (polyester, vinyl ester and epoxy resin) polyester resins generally have the lowest adhesive properties of the three systems. Vinyl ester resin shows improved adhesive properties over polyester but epoxy systems offer the best performance of all, and are therefore frequently found in many high-strength adhesives."

Epoxy is stronger:

Graph shows after 7 day room temp cure Epoxy has 6500 psi tensile strength, poly and vinyl esters have 5500.

"After a cure period of seven days at room temperature it can be seen that a typical epoxy will have higher properties than a typical polyester and vinyl ester for both strength and stiffness."

Epoxy is more water resistant:

"Both polyester and vinyl ester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures. As a result, a thin polyester laminate can be expected to retain only 65% of its inter-laminar shear strength after immersion in water for a period of one year, whereas an epoxy laminate immersed for the same period will retain around 90%."

Poly is probably fine for the OP's fan fairing but epoxy is better in every way except cost.

[David Tilston]

What I get from what you have said is that in a trailer application, (basically dry) polyester resin will have 85% of the adhesion of epoxy. Is the advantage adhesion only? What advantage does that provide? Adhesion to what? If it is better in every way, what benefit would the OP gain from using epoxy?

As I originally said, it is my understanding that the vast majority of the strength of fibreglass is in the glass. Could you please explain where epoxy has an advantage in a trailer environment.

I do not consider water degradation of polyester to be a problem in this application.

[MCDenny]

Originally Posted by David Tilston

What I get from what you have said is that in a trailer application, (basically dry) polyester resin will have 85% of the adhesion of epoxy. Is the advantage adhesion only? What advantage does that provide? Adhesion to what? If it is better in every way, what benefit would the OP gain from using epoxy?

As I originally said, it is my understanding that the vast majority of the strength of fibreglass is in the glass. Could you please explain where epoxy has an advantage in a trailer environment.

I do not consider water degradation of polyester to be a problem in this application.

The technical paper cited above notes three characteristics (adhesion (to everything), strength, water resistance) where epoxy is superior to polyester resin. Also, Epoxy can be had with slow hardener that gives up to a half hour of working time.

All those factors seem relevant to a trailer application. I'm sure the polyester the OP used is good enough and, if he was careful with surface prep and mixing, the fan fairing will last a long time but my point is that epoxy is better.

Wooden boats covered with fiberglass set in polyester resin often have the fiberglass delaminate after several years of expansion/contraction cycles. Epoxy has replaced polyester in that application virtually 100%.

[David Tilston]

A couple of things:
The link provided is a commercial for product. As such it's conclusions are suspect.
Most of the benefits described are for marine applications. These are not relevant to trailers.
My 40 year old Trillium is made with polyester resin. It seems to have held up well.

I fail to see a cost / benefit based reason to use epoxy resin. You have said, "epoxy is better". But you have not provided examples of how it is better, other then it's application in a boating environment. I don't own a boat. If polyester will last 40 years, why spend more? You seem to be addressing a problem that doesn't exist.

[Night Sailor]

Polyester resin does not bond well to wood. It lasts about five years. I know because I have had to re-tab every bulkhead in my boat. Not fun. I would never use polyester resin except when I'm working on a large piece and want to maintain the same coefficient of expansion. For repairs I mostly use epoxy. Given a choice is but an epoxy boat over a vinylester boat and pay the extra cost.

As for paint, While epoxy primer paint is harder to work with, and more expensive , the results are worth it. It is hard as a rock, lasts forever, doesn't peel and is easy to repair a small section. It can be sanded to flatten the surface , to remove stains, and buffed to as shiny as you like. I prefer a flatter look to a super high gloss. I love two part epoxy paints.

[D White]

Originally Posted by Night Sailor

Polyester resin does not bond well to wood. It lasts about five years. I know because I have had to re-tab every bulkhead in my boat. Not fun. I would never use polyester resin except when I'm working on a large piece and want to maintain the same coefficient of expansion. For repairs I mostly use epoxy. Given a choice is but an epoxy boat over a vinylester boat and pay the extra cost.

As for paint, While epoxy primer paint is harder to work with, and more expensive , the results are worth it. It is hard as a rock, lasts forever, doesn't peel and is easy to repair a small section. It can be sanded to flatten the surface , to remove stains, and buffed to as shiny as you like. I prefer a flatter look to a super high gloss. I love two part epoxy paints.

Polyester binds excellent to wood.... it will last 40 years...trailer people go ahead and use it. It does not work as well in a marine environment...Boat People, quit passing on your experience with boats and making the trailer people feel they are not doing the best for their trailer. Boats aren't trailers and epoxy is much harder to use, much more expensive, much more finicky and harder to learn. Polyester is easy to learn and most anyone can do a nice, long lasting repair by themselves the very first time. That is the point of fixing these fiberglass trailers...they last longer than most any other type of trailer construction except maybe aluminum. Boats are boats and trailers aren't...go give boat advice to the boat people. It doesn't apply here...Fiberglass Dave

[Night Sailor]

It bonds excellent to wood until it separates. I wish it would hold. Repairing failed polyester tabbing has been a nightmare for me.

Epoxy is no easier or harder to work with than any other hardener/resin.

Why use cheap material that won't last? The labor to repair it again is not worth it.

[D White]

Originally Posted by Night Sailor

It bonds excellent to wood until it separates. I wish it would hold. Repairing failed polyester tabbing has been a nightmare for me.

Epoxy is no easier or harder to work with than any other hardener/resin.

Why use cheap material that won't last? The labor to repair it again is not worth it.

You are basing your advice on a boat tabbing problem...these are trailers not boats and don't have tabbing problems. Why are trailer manufacturers still using polyester instead of epoxy? Because it works well and has for 50 plus years. The accepted repair material for these trailers is polyester, same material as they are made of. My advice is based on accepted repair procedures used throughout the industry and 38 years of professional experience using ALL resins including epoxy and many you have never even heard of. You ridicule accepted INDUSTRY standards based on nothing. I'll bet your polyester problems can even be traced back to YOU as far as materials choice and application.

Trailer people. There are several people on here that tell you the right way to repair your trailers. If ANYONE says that epoxy is the only way to go, don't listen to them. THEY DON'T KNOW WHAT THEY ARE DOING. You are not being cheap or doing wrong by your prized trailer. Polyester allows you to do as good a repair as any shop or repair facility by yourself in your driveway the first time without practice and without paying someone like me $125 a shop hour. It is not hard, or complicated and YOU CAN DO IT!
I am going to continually call out the epoxy only people. THERE IS NO REASON TO USE EPOXY UNLESS YOU JUST WANT TO....PERIOD
Fiberglass Dave

[David Tilston]

I have much less experience than Dave W. I have never tried epoxy, except when helping my dad put a layer of epoxy on his 45' mahogany on oak sail boat, good old 711:
Alden Designs - Hart Nautical Collection, MIT Museum - Designers of Fine Yachts

I can tell you though that using polyester resin is easy. I was happy with my first project, and I now have delusions of grandeur.

[Darwin Maring]

Fiberglass Dave. Used Poly and gauze on model airplane wings and it worked great. (Gauze as in the stuff you put on a cut)

I have a question: Epoxy and Polly - Which one will adhere to the other.

[D White]

Originally Posted by Darwin Maring

Fiberglass Dave. Used Poly and gauze on model airplane wings and it worked great. (Gauze as in the stuff you put on a cut)

I have a question: Epoxy and Polly - Which one will adhere to the other.

Well, this is where we get into trouble because we're dealing with molded trailers here only...I would seek advice on the model airplane forums for your best result....thanks....Fiberglass Dave

[Andrew Gibbens]

I will join Fiberglass Dave is saying that I don't see any reason to repair polyester with epoxy, if you are going to laminate the repair (ie, 'wet' resin and glass).

The one advantage of epoxy to an amateur is that it bonds better to existing fiberglass, if that hasn't been properly prepared. But since properly preparing existing fiberglass only means thoroughly sanding it where it will be bonded to, this isn't a great advantage.

If it's a small hole that will be plugged with some filler, epoxy putty does make some sense - those knead-and-use epoxy putty fillers are effortless, simple and strong. But then that's not lamination.

[Lora]

The OP 'cleaned up' the pop-up roof of my Compact II. He said a tree fell on it. There was lots of junk in the surface, so I got it down and went to work with the sander. I have found that there are numerous spider cracks and many old screw holes. And lots of what looks like bondo. There are many places where the gelcoat is gone.
He also used the material (bondo?) between the plates and bolts for the hinges and the roof and over the screws used to hold the wood to the roof.
I am concerned that the material will start falling out of numerous screw holes, and that the chipped gel coat and spider cracks around other screws will continue degrading unless I add some reinforcement (thru fiberglassing.)
My problem is, can I fiberglass over this bondo or do I need to remove all the old bondo? If so, recommendations for a good way to do that?
If the spider cracks are essentially repaired with bondo, should I just leave them alone?
Here are a couple of pictures of what I am dealing with.

[Bob Miller]

Just as a thought, as there are so many repairs and damage to cover, you might think about getting it sprayed with one of the better "Bed Liner" coatings such as "Rhino Liner"
Enter to Win Free Rhino Linings Gift Cards and Get the World's #1 Spray-on Truck Bed Liner
I had this on the bed of my Sonoma for five years and it still looked like new when sold.
You can get it in white or any color you want and I believe that they can add a flex agent to the mix. But be sure to ask how much weight it might add.

[Lora]

The weight is a concern. I was thinking about just doing a full layer of fiberglass mat, maybe from the inside, but I'm not sure how much weight that will add. Something else to consider is what that might do to a cover that I or some future owner might use. And, would that have to be sanded off, if I didn't like the look in the future?

[Bob Miller]

What ever else you might do to repair the damage may well leave some evidence anyway. It will just look like a textured paint and that's a sales plus because it seems even more impervious to weather than the original fiberglass. Plus they can custom mix any matching color you might like.


When I had my truck bed sprayed I have to sign a release that what was being sprayed on was "Permanent", so ferget removal. The stuff was originally invented to coat mining equipment in South Africa, it's, basically, last forever.

[Darwin Maring]

WEST SYSTEM | Projects | Fiberglass Boat Repair and Restoration - Polyester over epoxy

[D White]

Originally Posted by Darwin Maring

WEST SYSTEM | Projects | Fiberglass Boat Repair and Restoration - Polyester over epoxy

This applies to Gel coat (vinyl ester) only which no one hear has much access to and is for more experienced resin people. You have clouded the issue once again for people unnecessarily with something that DOES NOT APPLY TO THE AVERAGE PERSON TRYING TO WORK ON THEIR PRIZED FIBERGLASS TRAILER.