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Polyester vs. Epoxy.

I just finished trying out polyester resin after using Epoxy a couple of times. I tried Bondo because I had Autozone in-store credit. Granted this is not marine grade and I have read other people who said this was bad, but I thought if I kept it above the waterline it should work. I was replacing a 2x3 ft section in the bottom of my scorpion tub. This stuff was horrible although I almost made it work. I used 2 quarts and 2 layers of fiberglass. 1/2 way through the second layer my resin was too thick to spread. The directions said 15 minutes of until it sets. I am sure it was close to 5 when it transformed into a sludge. So I am left with this patch not quite done. I ended up finding and edge and pulling the whole 2x3 ft patch off. Next time I will use the epoxy.

Live and Learn the hard way.


Brian - I hate to say it, but it's more likely your technique that's giving you trouble. First thing I recommend is practice - and do it on something that's not your boat. Try cutting a 6" hole in a box and patch that for the learning experience.

All two part resins have a "pot life". When that hits, the chemical reaction changes the material and you're done with that batch. Epoxy has a longer pot life, but over catalyzing will shorten even it's working time. Temperature plays a BIG role. If it's a hot day you need less catalyst. With polyester resin you need to work in stages. Put on a layer and let it set. Put on the next layer and let it set and so on. Epoxy has a different approach.

By Bondo you do mean Bondo Brand Polyester? To us old timers Bondo was just a pink filler putty for clunker car body dents when you couldn’t afford to fix it right. I see Bondo Brand has a whole line of polyester resin products now so I trust you are using syrupy resin and not pasty putty.

Is your resin laminating resin or finish resin? Laminating resin stays tacky and a new layer can be applied directly, finish resin contains wax that rises to the surface and seals out air for complete curing. Finish resin can be layered if the wax is cleaned off and it’s sanded in preparation. Laminating resin will form a chemical bond becoming one piece when laid up within the given time period. This is how the boat was originally made.

Lay-up with finish resin only provides a adhesive bond between layers of work. This is probably the biggest misconception about polyester and epoxy. Epoxy only forms a adhesive bond, but it sticks better than a polyester resin adhesive bond BUT not as strong as a polyester resin chemical bond. This is why epoxy patches made of layers of fabric are done in one shot usually at the workbench and then fitted to the damage area, where polyester patches are more often laid-up in place right on the damage spot. Have you got a book on fiberglass boat repair – everyone doing this stuff needs a good book to refer to. The resin instruction sheets tell you the ideal situation, the books tell you all the variations like temperature that can cause differences in handling.

It seems as though you over catalyzed or it was hotter out than the ideal situation the instructions were written for. You appear to have tried to do too much at one time for the resin type and conditions. So, plan your steps, slow down, practice, try different test batches of resin for the time of day and temperature, and don’t go near you boat until you have a good feel for the materials behavior.
Thanks Dan. The first batch lasted about 15 minutes and the second batch only had a pot life of 5 minutes. They were both pint size containers and I used all of the hardener that came with each kit. If I had another chance I would lighten up on the hardener. My assumption about just using the portions provided in the kit was not very good. Outside temperature was about 75 F and low humidity. I did layout the glass on the boat and then wet each layer although, I did not wait to let it dry before working on the second layer.

I will stick:) with the epoxy next time, it has a pot life of 45 minutes. I need all of the time I can get. Maybe, I will even read a book of two.

On the positive side, my first two patches with epoxy were problem free.



Brian – “Kick” time that extreme appears out of the proportion if the temp is steady. I can only guess what looked like 50% of the MEK-P tube was under the mark on the first pass and over the mark on the second. Epoxy looks like your cup of tea.


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I use West System® epoxy exclusively today. There are three grades of catalyst to modify the set time. It is important to mix the ratios very carefully as to proportion.

I use disposible plastic trays, and disposible, sheet-metal-handled "solder brushes" for small batches. If I need additional time—or if it's really hot out—I put a slab of aluminum in the freezer and put the tray on that.


New Member
Dan's info about laminating and finish resins is right on the mark. I have repaired many old Chevy Corvette's and he is correct about how to use the resins. In automotive repair, epoxys resins are used for high strength joints where 2 pannels are joined together or to the frame.

One more point about fiberglass repair. A resin rich repair is not strong. You need to soak the patch to get a good bond to the substrate. Then you use a laminating roller or paint brush to remove as much resin as possible. Pull resin out of the area until it is very dry. It is dry enough when you pull out any more resin you will start to get air bubbles in your repair. If this happens add a bit more resin back.

I would also work in much smaller sections if possible. It is difficult to apply the repair, roll it flat and pull out excess resin on an area that large before the resin kicks.

You will also find that old/opened catalyst does not work in the same manner as new catalyst. If the catalyst has been opened for a while, toss it and use new. The strength of the bond varies with the resin to catalyst ratio. In my experience too much catalyst makes a brittle poor adhering repair.
You are right about age being a factor. One very old kit, did not even have a drop of hardener left after I cut open the container.

Possible my Autozone does not move polyester resin off the shelf very fast. I noticed they still have not replaced the resin that I bought when I first posted this thread.