Using smooth-cast 326 as "gel coat" for fiberglass casting

tubachris85x

tubachris85x

Master Member
Just as the title suggests...I am planning to do some cold-cast fiberglass cast pieces for a current project. I wanted to see if anyone else has used a urethane resin before as their gel-coat for fiberglass casts, and whether or not you had issues or difficulties in getting the process correct.

Thanks!
 
I've used Smooth On Epox-red as a gel coat, and separately I've used smooth cast 3 as a get coat for a matrix mold mother mold. It's worked fine both ways.
 
chukzilla said:
I've used Smooth On Epox-red as a gel coat, and separately I've used smooth cast 3 as a get coat for a matrix mold mother mold. It's worked fine both ways.
Click to expand...

So do you think it won't be an issue applying fiberglass resin and matte during a urethane "gel period" as you would any other fiberglass process?
 
The ideal people to ask would be Smooth-On.

There is not much information in your question, so I will ask: Do you intend to use the same 326 urethane resin with the fiberglass? Do you intend to use another type of resin: Polyester. Epoxy?

Urethane, once cured, does not accept additional layers well. (that is layers of Urethane, Polyester, or epoxy) They all tend to de-laminate.

As a general rule: Don't mix materials. If you are going to use urethane as a surface coat, back it up with more urethane resin. The second layer will adhere better if you give the first layer a sanding. It will also ruin a brush every time you use on, because you can't clean cured resin out of a brush. (from the spec-sheet, 326 has a pot-life of 9 minutes)

If you are going to use polyester resin for the fiberglass, also use polyester gelcoat.

Epoxy resin (recommended) doesn't need a gelcoat, but is more difficult to laminate than polyester.

Lastly, Vinyl-ester resin is a nice half-way point between polyester, and epoxy.
 
clonesix said:
The ideal people to ask would be Smooth-On.

There is not much information in your question, so I will ask: Do you intend to use the same 326 urethane resin with the fiberglass? Do you intend to use another type of resin: Polyester. Epoxy?

Urethane, once cured, does not accept additional layers well. (that is layers of Urethane, Polyester, or epoxy) They all tend to de-laminate.

As a general rule: Don't mix materials. If you are going to use urethane as a surface coat, back it up with more urethane resin. The second layer will adhere better if you give the first layer a sanding. It will also ruin a brush every time you use on, because you can't clean cured resin out of a brush. (from the spec-sheet, 326 has a pot-life of 9 minutes)

If you are going to use polyester resin for the fiberglass, also use polyester gelcoat.

Epoxy resin (recommended) doesn't need a gelcoat, but is more difficult to laminate than polyester.

Lastly, Vinyl-ester resin is a nice half-way point between polyester, and epoxy.
Click to expand...


I suppose it wouldn't hurt to use the same resin throughout the whole process. The 326 I'm using has a 60 minute cure time, so I figured once it starts to become tacky/gel, I can go forward applying additional resin and fiberglass matte.

I know this isn't the ideal material to use, but due to my time constraints and general materials knowledge on the subject, I can't spend a lot of time trying to experiment since I need to do cold cast pieces- which I already know how to do properly with the 326
 
Interesting thing.

I had purchased Free Form Air from smooth-on a couple years ago. Never opened it, actually got it for making the mother molds for this project. I was doing some digging around and it's explained that the FFA is able to be used as a laminate for fiberglass cloth....so I am going to give this stuff a shot. The stuff I have is NOT the liquid-mix version, it's the straight up 2 part putty, just how magic sculpt is, but perhaps this will be much lighter.
 
I would recommend using an epoxy resin for your fibreglass backing. It won't create a chemical bond like when using similar materials, but it will create a very strong mechanical bond, especially if you rough up the surface of the 326 resin before applying it. I have no experience of applying any fibreglass to a still tacky urethane resin, only cured stuff. It's not ideal, but as you point out, sometimes on a time crunch you just need something that will work.

Worth pointing out that you can't use regular fibreglass mat with epoxy resin. You either need a powder bound mat that is specifically for epoxy, or the woven cloth.
 
I made a few phaser shells by slush casting Smooth-On Onyx resin as a gel coat and then backing it with mat and epoxy, and I had bad problems with delamination. The epoxy, Smooth-On Epoxamite, just refused to bond to the Onyx. I went to an all epoxy method, using either Epoxacoat or Epoxamite thickened with Cabosil to gel coat with.

It may be possible to wet the glass with the 326, though from what others have said it may also have problems bonding if the first coat has set too far. The epoxy will work even if the gel coat has cured completely, it bonds like it's one piece. Any boat building supply store should have epoxy and Cabosil, I go to West Marine in Woodbridge VA, they also have mat that works with epoxy.
 
Re: Using smooth-cast 326 as "gel coat" for fiberglass casting

robn1 said:
I made a few phaser shells by slush casting Smooth-On Onyx resin as a gel coat and then backing it with mat and epoxy, and I had bad problems with delamination. The epoxy, Smooth-On Epoxamite, just refused to bond to the Onyx. I went to an all epoxy method, using either Epoxacoat or Epoxamite thickened with Cabosil to gel coat with.

It may be possible to wet the glass with the 326, though from what others have said it may also have problems bonding if the first coat has set too far. The epoxy will work even if the gel coat has cured completely, it bonds like it's one piece. Any boat building supply store should have epoxy and Cabosil, I go to West Marine in Woodbridge VA, they also have mat that works with epoxy.
Click to expand...

That's funny, I live in Woodbridge, Va. I'll have to check it out, haven't been to a west marine since I left Florida

- - - Updated - - -

bookface said:
I would recommend using an epoxy resin for your fibreglass backing. It won't create a chemical bond like when using similar materials, but it will create a very strong mechanical bond, especially if you rough up the surface of the 326 resin before applying it. I have no experience of applying any fibreglass to a still tacky urethane resin, only cured stuff. It's not ideal, but as you point out, sometimes on a time crunch you just need something that will work.

Worth pointing out that you can't use regular fibreglass mat with epoxy resin. You either need a powder bound mat that is specifically for epoxy, or the woven cloth.
Click to expand...

That's something that I have thought about because I see no real reason why it wouldnt physically hold itself to the back of the urethane.
 
Best way to do a urethane gel coat with fiberglass is to dust the urethane with cotton flocking while it is still tacky. This will supply the fiberglass resin with a strong mechanical tooth to bond to in place of a chemical bond. Works with all the urethane resins I've tried it with.

Learned this courtesy of another contributor to the forum here when I was trying to figure out how to do the same thing.
 
Ozymandius said:
Best way to do a urethane gel coat with fiberglass is to dust the urethane with cotton flocking while it is still tacky. This will supply the fiberglass resin with a strong mechanical tooth to bond to in place of a chemical bond. Works with all the urethane resins I've tried it with.

Learned this courtesy of another contributor to the forum here when I was trying to figure out how to do the same thing.
Click to expand...

Interesting, nothing special to the process? Throw the flock down while tacky and then I could apply something like an epoxy/fiberglass over that after the urethane is cured?
 
Gonna be finishing up the molds tonight, and should be ready to start casting tomorrow morning..

Getting anxious about this as I've only got tomorrow to produce workable casts for this suit...
 
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