Poly over Epoxy

[LMB]

It seems to be commonly accepted that you can't put Polyester Resin over epoxy. Why? or is this not entirely true. I've applied polyester based gel-coats over epoxy many times by allowing the epoxy to fully cure and achieve a secondary bond thru mechanical abrasion. Never a problem and West System advises this as an acceptable method.

Once epoxy is fully cured doesn't it become "inert" ? If so, you should be able to apply virtually any material over it with proper prep.

One more question? How does Vinylester fit into this compatability equation. I know it mixes well with poly. In the technical documents vinylester has some sort of epoxy component but since I'm not a chemical engineer I'm trying to make some common sense out of all this.

I've had a hard time getting anything to bond well to vinylester. The epoxy can have amine blush that will prevent poly adhesion. SB112 doesn't blush and I have gelcoated over that. I'm sure there are guys here better able to explain alternatives and the "whys". I think why the poly is felt not to adhere to epoxy is that the secondary bond of epoxy is stronger than the primary bond of poly, so the delam will always happen at the interface, hence the notion that poly doesn't stick to epoxy.

[Steve W]

Your right, it is often stated on this forum that poly wont stick to epoxy which is not true, while the blush can prevent the poly from curing at all, like most things its all about the prep, get rid of the blush, abrade and it will cure and bond, how well though i cant say as ive never tested the bond but as mark said,the epoxy bond will be the stronger bond by far so i cant think of a situation where you would want to apply poly over epoxy. My only experience with this is when i manufactured fiberglass dinghies many years ago we glued the wood gunwales on with epoxy and the center thwart cleats and occasionally a bit of epoxy got onto the hull and if it wasnt cleaned up the gelcoat we painted the inside with wouldnt cure at all on that spot, if we cleaned it with alcohol no problem. I also once owned a production built cold molded 24ft sailboat with a beamless laminated plywood deck which was sheathed with dynel/epoxy and then it had granulated cork on gelcoat for non skid,i have no idea why he used gelcoat over the epoxy but it worked just fine so clearly the prep was good. I wouldnt do it.
Steve.

[PAR]

It's not that polyester doesn't stick to epoxy, it just doesn't stick well, particularly in comparison to polyester on polyester and especially compared to epoxy on polyester.

The amines prevent the cross linking of the polyester molecules during cure. Water is the preferred solvent for the removal of amines. When using west and some of the other modified epoxies the bond will be fine. However, the flexural differences between the polyester and the epoxy can be a problem. The addition of vinylester additives to the poly will help with this.
We've been doing epoxy repairs with polyester gel coat finishes for 25 years with no problems. However...... the addition of vinylester to polyester gel coats will enhance the color and gloss retention of the repair, while the surrounding area will lose gloss and color at a normal rate.

[LMB]

O.K. that's what I thought. As I continually re-educate myself I have to revisit these issues from time to time. I would not attempt to combine the two in a structural application however I occasionally find a situation where there is the need to combine the two. An example would be the shaft log repair I mentioned in another post. Here, we are dealing with a polyester built small boat, with an all polyester structural rebuild. As I sometimes opt on certain repairs I have elected to bed the shaft log in G-flex and glass in with 1808 and epoxy resin. Once fully cured, I planned to properly prep and finish the remaining bilge with an additional layer of glass and gel-coat. Obviously, some of the poly will overlap onto the epoxy repaired area and then there is the gel-coat application. If FG1 is right about vinylester then it would be a good choice for tie coat situations. I suspected this (about vinylester) but I also thought I recalled reading something about an incompatibility of the two. Maybe that was just referencing the more chemical incompatibility of primary bonds and not secondary bonding applications. If what I'm interpreting is correct, polyester or vinylester can achieve just as good secondary bond to epoxy as it can itself. Again, I'm talking secondary bonding to fully cured substrates. Not debating which achieves the best bonds just looking at acceptable alternatives as their application arises in cost effective repair options. I keep all these resins on hand so its a matter of choosing the best application.

I'd like it if somebody that knows says something about getting anything to stick to vinylester.

[War Whoop]

Originally Posted by mark775

I'd like it if somebody that knows says something about getting anything to stick to vinylester.

What is the problem? it is done everyday building VE boats,Excellent secondary bonding.

[LMB]

I'm actually a big fan of vinylester. I've used a good deal of it in repair work. I typically use vinylester based fillers and fairing materials as well. I've topcoated with both gel-coat and epoxy primers. Never a problem, in fact consistent and excellent results. If you look at the technical papers for vinylesters you'll find performance is actually better than epoxy in some instances.

[Fanie]

Polyester resin will work if you encapsulate the item so delamination cannot take place. Even on largish surfaces a nice hard pluck starts to tear it off.

OR you can glue the piece of polyester glass in place if you have enough surface - which will be my first choice if you join pieces. If you mount a steering console on a deck for instance, then just glass it you can break if off. If you allow a foot say 50mm all round the base and you glue it with sikaflex or similar sealants (just not silicone ! ) then it will be really hard to break if off. You can however remove it by sawing the sealant off with braid or piano wire that will cut it.

The same law applies when you make something from polyester - it has to be ONE PIECE or you have to join it with an alternative method.

[Fanie]

I'd like it if somebody that knows says something about getting anything to stick to vinylester.

Well Mark, here's one that does work ! The glue you see is a Dow Corning sealant, it's about 60mm wide there and if we support the surface nearest to you on a pivot then 200kg's doesn't tear the joint apart. The glass begins to bend and the sealant on the edge stretch a little but it holds well.

Make a test piece and see how well it works. Of course you don't put it on painted surfaces or it won't dry (anytime soon). The sealant cured on the raw glass, I think I left it a few days.

[War Whoop]

Here is a Vinylester hull with 1' Corecell bagged in Corebond (polyester putty) NO Problem if you read the can!








Inside Vinylester Laminate going down






Finished boat running.



Where is yours?

[Fanie]

Dammit War Whoop,

I was staring at the last picture for 5 mins before I realised it was a BOAT on the water...


Cupela things on the paint job. They were in a hurry to get on the water, look how the paint smeared in the wind and water









Nice looking boat !

Ugh.
Thanks, Fanie.

[War Whoop]

Originally Posted by Fanie

Dammit War Whoop,

I was staring at the last picture for 5 mins before I realised it was a BOAT on the water...


Cupela things on the paint job. They were in a hurry to get on the water, look how the paint smeared in the wind and water










Nice looking boat !

LMAO and Thank you.