Finishing polyester resin woes

[Kez]

Rebuilding an old 40 vee and glassed in new engine rails. All went well and time to seal the rails with finishing resin. I do not like the Bondo stuff for autobody work as it is too thick. So I bought this marine finishing resin from West Marine. It flows much like the Hobbypoxy resin, which I like.

I mixed up a small batch with the recommended catalyst (12 drops per ounce) and it kicked off literally in three minutes. So I reduced the catalyst to 8 drops and it seemed to give me enough working time. But a day later, it is still tacky and soft. I can see the wax floating onto the surface and the left over resin cured hard. But the resin on the rail is sticky.

I see two options now. 1. Apply another coat of resin with a lot of catalyst and hope that it will cause the previous layer to cure. But the resin will be too thick and hairline cracks will develop. Also the mount will not fit without sanding.

2. Try to sand or scrap off the uncured resin and start over. But it will probably clog the sand paper.

Anyone has better ideas?

Thanks. kez

 

[MarkScott]

What are you mixing this in? I recall years ago I was mixing resin in plastic, only to have what you describe. Once I looked that the hardner bottle, it clearly states not to mix in plastic. Don't know if this is the issue, but could be.

 

[Hydro Junkie]

Are you sure it was poly resin and not epoxy? If it's the epoxy, I've used West Systems for years and, the only time I've had issues is when I played with ratios or put my project in an unheated garage. Here's the write up for West systems from their website:

Recommended Temperature Range for 207 Hardeners is 60F to 90F, at 72F the 207 hardener has a 20-25 minute pot life, keep in mind that epoxies set up faster in large masses or hot temperatures.Recommended Applications for WEST 207 Hardener are fabric application and wood coating where a crystal clear finish is desired. Such as wood strip or stitch and glue boats.

 

[dave roach]

I use the 207 and never had a problem with it.The pumps that you get for the west system is one pump of the 105 and one pump of the 207.

Dave

 

[HTV Boats]

The wax on the surface seals resin from air which retards the cure. I have had gel coat stay tacky on outside repairs. Try some PVA (poly vinyl alcohol) over the resin and see if it will fully cure. PVA helps polyester cure similar to wax in resin.

Mic

 

[bzubee]

Or hit it with a heat gun! Unlike epxoy heat helps speed up the cure time of resin.

 

[Kez]

Thanks all for your replies.

Mark, it is true that polyester resin will not cure over certain materials, for example, epoxy. I use the container that is included. And the left over resin cured properly in the container.

HJ, I am sure it is finishing polyester resin and not epoxy. In fact, epoxy has no problem curing over polyester resin. But resin does not cure over most epoxy.

Mic, I heard of adding PVA to polyester resin to help it cure but I have never tried it. I think the catalyst (MEK peroxide) will evaporate at the surface so they add wax in the resin which floats to the surface to prevent the MEK from evaporating. At this point adding PVA or wax will probably not help since there is no more catalyst to cure the resin.

Bill, I will try heat gun but I am not sure if it will help for the same reason. If there is no more MEK in the resin, heat may not help very much. But I will try it.

kez

 

[HTV Boats]

You don't mix PVA with resin. After things go from liquid to the gel stage and start to get solid spraying a layer of PVA on the outside helps the cure. I have had parts stay gooey from not mixing long enough and catalyst that went bad also. Mixing a batch of hot resin and paint over can stimulate cure. Heat is the key and Bill is correct there. If the part is small enough to box in with light bulbs as a heat source it may cure in a couple days.

Mic

 

[bzubee]

Or put it in your the trunk of your car parked in the sun or up in your attic for a day or so!

 

[Bob Kensill]

UV light will make it cure

Sent from my VS985 4G using Tapatalk

 

[Blaise Recca]

Sunlight, bright and hot. Been there, done that.

 

[Kez]

Mic, interesting idea of putting PVA on polyester resin to help it cure. I guess it works much like the wax in the finishing resin. I will try it next time for sure. Actually, I used to put cellophane on a flat surface and the resin will cure hard. It works well if you want to seal a flat surface such as a radio box.

Bill, I want to cure the tacky resin first and I will try applying heat tomorrow.

Bob and Blaise, thanks for sharing your experience. Sunlight has both UV and heat. We will hit 92 tomorrow so I will let the boat sit on the deck and will report the results.

kez

 

[Kez]

Update: I left the boat in the sun yesterday. It was hot and humid. After 4 hours the surface became less tacky but the resin is not as hard as it should be. I can easily put a fingernail mark in it. So I am not sure if the finishing coat will be as fuel proof has it can be

I am now thinking it is probably the high humidity that caused the resin not to cure hard. Another possibility is that the acid brush I used to apply the resin has some contaminants on it.

I will wait for the resin to cure hard enough so that I can sand it and I will recoat. I will test the PVA trick on a piece of scrap.

 

[SayMikey]

You will wait the rest of your life. Hobby poxy and polyester dont like each other never have and never will. Get a razor blade and start scraping. West works ok. The rule is epoxy will cure over cured resin but resin will not over epoxy

 

[Kez]

Mike,

I am aware that polyester resin does not cure over epoxy. That is the reason why I prefer to use polyester resin to bond the rails. There is no epoxy in the hull. I mentioned Hobbypoxy in my post as I liked the resin they used to offer.

I think the problem is humidity. I stored my resin in the basement. When I worked on it outside in humid condition (dew point 72 degrees), the cool resin probably attracted enough moisture and affected the curing. I will have to sand or scrap it off and start over. I will try the PVA trick on a piece of scrap material.

kez

 

[Mike Luszcz]

This has happened to me. Whenever you put polyester resin on bare wood, it tends to stay tacky if the coat is really thin. I have learned to really add a lot of resin over it as that will let it cure. Something with the wood, it must soak out the MEKP. The best way to seal wood is something like West Systems or US Composites 635 epoxy. Laquer thinner will break down that sticky surface for you tho, but you have to really wet it heavy on a rag to get that to work well. Mike

 

[Patrick Mulvany]

I just had this happen to me laying some resin on bare wood and it stayed tacky for four days. I used acetone to peel the resin back off. I think I had some old hardener because it's 90 degrees here in Fl. every day.. Will continue with fresh hardener and yes I use West resin!!

Later!!

Pat

 

[Mark Poole ModVP]

I have had some trouble with outdated resins and catalyst from the retail chain that you bought it from. I bought resin and gel coat from them that was out dated and it did not cure.

The Bondo brand polyester resin may be a little thicker than most but I have never seen a batch fail to cure if mixed right. It wets out light weight csm and cloth nicely. It is too thick for wetting out the heavy cloth like they build life size boats with.

It is a good idea to 'hot coat' wood with a coat of resin and let that dry before glassing over.