EPOXY QUESTION

[Chuck Feldman]

There is Epoxy and there is Polyester epoxy. Epoxy is generally mixed 50/50. Polyester is mixed with an activator that is a few drops to a small dab of the polyester resin. OK most of us use epoxy like we get from the hobby shops, 50/50. I do to. But now I am using a finishing resin that is polyester. It mixes as above, afew drops. Question is this; can you apply polyester over regular epoxy? Can you apply regular epoxy over polyester? Another question is Do they make a regular epoxy finishing resin? Thanks for your help on this. Chuck

[Leester]

I have Z-Poxy finishing resin the hardener states (contains polyamide resin) a caution on the tube says it may not be compatable with polyester resin.

[kvarley]

In my experience ,you can not use polyester resin over epoxy . For some reason the polyester will not harden in that area , and remains tacky for ever.  Keith Varley

[john e. holliday]

I used to use the Polyester resin, the stuff that uses a couple of drops of activator.  The stuff was hard for me to sand.  That is why I went to epoxy for the base coat when putting on glass until I discovered the water base polyurethane clear paint.  Do not even think of putting polyester resin over epoxy finish.  But, you can put epoxy over poly when it has cured and been sanded.  The epoxy paints go on much thinner and are easier to use in my opinion.  Have fun,  DOC Holliday

[Will Hinton]

I love the Z-Poxy finishing resin - it is easy to sand, really has a good strength for a finishing resin, and fills wonderfully.  The luthier crowd is even starting to use it as a primary grain filler on really porous woods before applying Nitro-cellulous lacquer because of the ease of sanding.  I just used it to apply carbon fiber vale over some molded balsa booms for my new Ephesian twin boomer and the strength gained is quite superior to just doping it on.  I can use much lighter balsa for my molding this way and still have the strength.  (I used 3/32 x  4 lb stuff on these.)  There is also no smell involved!!!!
Blessings,
Will

[Robert McHam]

This thread has a lot of interest for me.

First I need to say that I have not tried polyester type epoxy, therefore I do not have any hands on experience applying over the regular 50/50 mix type.

However...
My understanding of epoxies is that they essentially perform a mechanical bond to whatever it is applied to. To me, that means that it doesn't matter what is applied to but in order that it bond well there must be a rough surface or it will peel off. A simple light sanding usually suffices, as one might sand between coats of paint. Smooth surfaces present a huge problem with adhesion.
The fact that one mixes the epoxy with a hardener implies that the "cure" is chemically attained as opposed to drying out the solvents like most of the other glues we use. This cure should occur whether in the presence of air or not.

Since the first coat of one epoxy should be cured before an application of another type, chemical hindrance between the two types should not be an issue. Or is it in these cases?
Is the polyester chemically reacting in a negative manner to the 50/50?

If so, could this be thwarted by using some other substance between the two, such as a thin coat of polyurethane? Yeah, I realize this is not a good answer for several reasons but it is merely a suggestion.

Robert

[billbyles]

First I need to say that I have not tried polyester type epoxy, therefore I do not have any hands on experience applying over the regular 50/50 mix type.

However...
My understanding of epoxies is that they essentially perform a mechanical bond to whatever it is applied to. To me, that means that it doesn't matter what is applied to but in order that it bond well there must be a rough surface or it will peel off. A simple light sanding usually suffices, as one might sand between coats of paint. Smooth surfaces present a huge problem with adhesion.
The fact that one mixes the epoxy with a hardener implies that the "cure" is chemically attained as opposed to drying out the solvents like most of the other glues we use. This cure should occur whether in the presence of air or not.

Since the first coat of one epoxy should be cured before an application of another type, chemical hindrance between the two types should not be an issue. Or is it in these cases?
Is the polyester chemically reacting in a negative manner to the 50/50?

If so, could this be thwarted by using some other substance between the two, such as a thin coat of polyurethane? Yeah, I realize this is not a good answer for several reasons but it is merely a suggestion.

Robert

Hi Robert,

Polyester resin is not an epoxy; the two are chemically quite different.  There are two kinds of polyester resins...there is the original polyester resin, and the later vinylester resin, both of which use the Methyl Ethyl Ketone Peroxide catalyst.  The MEKP catalyst merely causes the polyester resin to cure and is not part of the final result.  With epoxy there is part A which is the resin and part B which is the hardener and they are almost always used in a 50-50 ratio.  This reaction forms long chain polymers and both parts A & B are part of the final product.  Polyester, vinylester, and epoxy resins cure as a result of chemical reaction and do not rely on air drying of solvents.

You are correct in that epoxy requires a mechanical bond, the underlying surface must be sanded or scuffed with 3M Scotchbrite (typically the red pads #7447) in order to provide tooth adhesion.  Epoxy can be applied over polyester with no problem, however polyester should not be applied over epoxy. 

Bill Byles

[Howard Rush]

I had the same experience as Keith: the polyester won't harden in those places where there's epoxy glue.  Oddly, if you covered the epoxy glue with epoxy paint such as K&B Superpoxy (now extinct, boo hoo), the polyester would harden over the epoxy paint. 

I'd use something more up-to-date than polyester.  I'll have to try Z-Poxy.

I used to use polyester resin a lot.  One thing I learned was that you could delay its cure by putting it in the freezer, but it made everything in the freezer taste like styrene monomer.  That's not my favorite ice cream flavor.