Poly over Epoxy what West System says....

[BillP]

Re: Poly over Epoxy what West System says....

I am talking here about regular poly resin (finishing or laminating), not special resin made for thickening. Sagging is caused by the resin separating from the spheres and basically all common "thickeners" do it. When cabosil alone (or any other "thickener") is mixed high enough to prevent sags it makes a soft, weak compound (you might as well use baking flower from the pantry). Using a combo of microballoons or glass fibers WITH cabosil is the stronger material with best sag control. As advertised, Cabosil helps bond everything together.

BUT, here we go again with an application for accelerated catalyst...it speeds up kick times and greatly reduces sagging (zeros it out if you know how to ratio accurately). So the question is...for sag control do you want a super weak mixture catalyzed at normal rates or a strong mixture catalyzed over the "suggested" ratio? Heating isn't a substitute for this.

Last but not least...stiff but flexible mylar sheet applied over the patch can control sags while giving a smooth finished surface when peeled off.

bp

 

[Mark42]

Re: Poly over Epoxy what West System says....

Thanks for that advice, Bill. I think to reduce sag, I will just work with gravity, not against it. So when the sides are being faired, the top will just be turned on its side so the surface being faired is horizontal, not vertical. Shouldn't be a problem. Also, it does not look like the top will take too much fairing, hopefully not more than half a gallon mixed.

I ordered some thickener today, so the 7" sander and thickener should come about the same time. Hopefully before this coming weekend. Still have a few weeks of warm weather before its below 50 in the afternoon (I hope).

 

[BillP]

Re: Poly over Epoxy what West System says....

Hey, those ez fibers look like like microfibers and might be a real winner for thickening. I've been using commercial grade microfiber towels for a couple weeks and its amazing how grabby they are for everything. The way they work is the fibers have multiple micro size split ends which pose more surface for stuff to cling to.

bp

 

[Mark42]

Re: Poly over Epoxy what West System says....

Bill, I ordered the ez fibers. I'll let you know how well they work.

 

[i386]

Re: Poly over Epoxy what West System says....

Thanks for that advice, Bill. I think to reduce sag, I will just work with gravity, not against it.

No harm in that strategy Mark, but I think you will find that once you get your stuff mixed correctly it will be a non issue.

 

[Mark42]

Re: Poly over Epoxy what West System says....

Interesting update.... I wrote to West Systems asking if they thought Bondo would work well as a fairing compound on epoxy. Got the response below. Basically, they say "no", do it at your own risk, although they still believe that gel coat over epoxy is OK, but again, at your own risk.

Its a moot point now anyway, because I'm using epoxy fill fillers for fairing.

Dear Mark,

Bondo is a filled polyester system so you?re correct in assuming Bondo will cure and bond to a properly prepared epoxy surface. The problem is that Bondo will shrink a great deal during its cure ? as much as 5% or more. This shrinkage will stress the bond to the epoxy surface. A similar thing happens with gelcoat too, but it is applied relatively thin compared to most fairing applications and generally covers a lot of surface area compared to its applied thickness. I have used Bondo and patching compound over epoxy to fill small voids, pinholes, etc. so the application thickness was very thin and it seemed to work. I also used Bondo to repair the front of my keel after hitting bottom because I was only able to hang overnight I needed the fast cure time ? that was years ago and its still there although it was bonded to the lead primarily.

With those issues in mind you should use your best judgment if this is the way you want to go.

Regards,

XXXXXX XXXXXXX (name removed for post)

Tech Advisor/Chemist

 

[ondarvr]

Re: Poly over Epoxy what West System says....

That's kind of the same response I got whe I called and discussed the bonding subject, "it works, but". In my testing I found the same thing, it works, but may not do it well depending on the situation.

 

[BillP]

Re: Poly over Epoxy what West System says....

Bill, I ordered the ez fibers. I'll let you know how well they work.

Mark42...have an update on the ez fibers yet?
Regards,
Bill