tie coat for gel coat over 2-part epoxy?

leakyrivot wrote:

For tie coats I found they reccomend the following in their FAQ's:

System Three wrote:
Can I put polyester gelcoat over cured epoxy?
In general, polyester resins won't cure properly or bond well to epoxy resin products without a "tie-coat" barrier resin in between. System Three SB-112 resin system can be used as a tie coat in between epoxy laminating or coating resins, and polyester laminating or gelcoat resins. Using this resin allows you to gelcoat an epoxy-built or epoxy-repaired boat.

Jon

Good find.
Here's ashot of my new dash board...

Normally the only obstacle to a proper Gelcoat over epoxy cure is the amine blush issue. Amine blush is the residue that follows a cureing of epoxy and inhibits the sticking of anything to the epoxy coat finish. As such, you need to remove the amine blush, which can be done with a scotchbrite pad and some elbow grease. Once the amine blush is removed you can lightly sand the surface, wipe with solvent and gelcoat not problem.

One caveat is to make sure the epoxy is FULLY cured. Epoxy, especially in cooler weather takes a while to cure, so make sure you wait a while and allow ALL of the amine blush to come out. Actually you may be better off letting it cure, cleaning, letting it sit some more, cleaning and then gelcoating. But, the bottom line is that you can gel over epoxy. Once cure its like any other fiberglass.

BTW I noticed you mention something about temp not being a problem for Epoxy. Thats not true, you need at least 50 degree farenheit for epoxy to cure properly..and the warmer the better.

Poly can cure down into the 40's with a touch of extra hardener (but not too much)

This post edited by GradySailfish 02:58 PM 02/08/2008

GradySailfish wrote:

Normally the only obstacle to a proper Gelcoat over epoxy cure is the amine blush issue. Amine blush is the residue that follows a cureing of epoxy and inhibits the sticking of anything to the epoxy coat finish. As such, you need to remove the amine blush, which can be done with a scotchbrite pad and some elbow grease. Once the amine blush is removed you can lightly sand the surface, wipe with solvent and gelcoat not problem.

One caveat is to make sure the epoxy is FULLY cured. Epoxy, especially in cooler weather takes a while to cure, so make sure you wait a while and allow ALL of the amine blush to come out. Actually you may be better off letting it cure, cleaning, letting it sit some more, cleaning and then gelcoating. But, the bottom line is that you can gel over epoxy. Once cure its like any other fiberglass.

BTW I noticed you mention something about temp not being a problem for Epoxy. Thats not true, you need at least 50 degree farenheit for epoxy to cure properly..and the warmer the better.

Poly can cure down into the 40's with a touch of extra hardener (but not too much)
Would a good sanding be ok? I've taken off a good top layer of the epoxy/fiberglass.

Although the above recommendations may produce a satisfactory result, they are far from what one might beleive he is acheiving.

Certain cured epoxies do become coated w/ an amine blush in humid conditions, or some even just when exposed to air during the curing process. You can easily wash the amine blush off w/ soap and water, which will prepare it for conventional coatings. However, most epoxies use an amine based hardener (which causes the amine blush),and this type of product will always have free radical amines on the surface of the coating. The amines will inhibit the curing process of a polyester resin, basically preventing complete cure of the polyester immediately at the surface of the epoxy. The Gel coat will appear to cure fine, but unknown to the owner, there is not a complete cure at the interface, and the only thing tying the two producs together is either the deep groves from sanding, which the cured portions of polyester may bite into, and the overall cohesion of the gel coat, which will maintain integral coaitng until a chip or crack develops.

Onward, remember that the greatest appeal of gel coat is that it will chemically and mechanically bond w/ the structural components of the polyester/fiberglass construction. When applied to epoxy, the best one could hope for, even disregarding amine inhibition, is a mechanical bond. Gel coat is brittle, and thick by application, so w/o the chemical reinforcement of the resin/fabric behind it, it is more liable to fail.

As mentioned before, epoxy is more temperature sensative than polyester. Even though there are epoxy formulations developed to work in temperatures below forty degrees, they are quite viscous in the lower temps, and difficult to work w/. They are also susceptable to high humidity, and the cure time will double w/ every ten degree temperature drop. Polyester can be treated to work in virtually any temperature.

If you can get epoxy to cure at a given temperature, you will have no problem overcoating it w/ Awl Grip, or any other premium two-part package. It will maintain it's color and shine longer, is easier to recoat, and will look a thousand times better than a gel coat that wasn't applied into a mold.

Paul

Gradysailfish wrote:
You can as dorymate said use dishwashing soap, or you can use a scotchbrite pad. Your choice.

Or both.

Capt Paul wrote:
If you can get epoxy to cure at a given temperature, you will have no problem overcoating it w/ Awl Grip, or any other premium two-part package. It will maintain it's color and shine longer, is easier to recoat, and will look a thousand times better than a gel coat that wasn't applied into a mold.

Thats what I would do unless you are **** bent on using Gelcoat. Try Interlux Perfection with roll and tip method.

This post edited by Dorymate2530 09:23 PM 02/09/2008

blush problems...

Why not just just MAS products, low viscosity resin and slow hardener? No blush. No curing problems. Will take primer/paint, varnish, readily. Good stuff to work with.

Been using it out in the garage all winter. I keep it above 50 degrees whenever feasible when putting down a lot of it, starting the bigger jobs early on Saturday to make the most heat all weekend long. Sometimes the overnight chronic temp drop cannot be helped but this has not created any problems for me. Not being in a hurry for a big winter project, not a big deal to wait and extra day here and there to ensure proper curing.

I hear they have come out with a faster hardener that is also non-blushing.

It is probably not likely the best product, I don't know much about glassing, so the MAS products are great for beginners to use (when building a 16 foot long glass over wood kayak [a mistake would be very costly]), being it is generally not a problematic product and compatible with a multitude of top coatings.

One other thing of note is we only wipe this stuff down with laquer thinner and let it evaporate for at least 45 minutes. Acetone and mineral spirits have too many impurities to use for the wipe down.



This post edited by Fishbust 06:58 AM 04/05/2008