by Wobbly Wheels » Sun Apr 10, 2016 10:05 pm
'Conventional wisdom' says polyester and epoxy resin systems aren't compatible, but 'conventional wisdom' also says you can catch a cold by being cold or that carrots are good for your eyesight. Sometimes things get repeated so often that become a truth unto themselves.
Fortunately for us, they are indeed compatible.
The problem with laying poly over epoxy comes when the amine blush (which isn't compatible) is not adequately removed from the surface of the epoxy and contaminates the bond. Fortunately, that blush is water-soluble and easy to remove...but we need to be thorough about it.
Going the other way, epoxies have long been used to repair boats laid up with polyester based resins. A barrier coat of epoxy based paint (like InterProtect 2000E) is applied over the (vinylester-based) gelcoat of virtually all new boats below the waterline before antifouling is applied...but again, the hull has to be thoroughly cleaned of any residual wax left over from the mold. Epoxy can encapsulate and form a mechanical bond with almost any substrate except soft, waxy plastics.
One of epoxy's virtues is that it is friendly to so many different materials. That's why it can be mixed with many different additives. Milled fibers (Cabosil), chalk and wood flour will give you a strong structural filler that will hold a screw and fillers like talc and microballoons provide a softer, lighter filler that is easily sanded. Epoxy doesn't shrink as much as polyester does as it cures, so you may want to use epoxy to mix up your own filler rather than having to make two passes the way one typically does with poly-based fillers. Then again, the cure times are so much shorter with poly fillers that it's probably more an issue of cost than time. You could also use wood filler or wooden plugs.
Coincidentally....starting on page 13 of the most recent issue of Epoxyworks (the West System publication) there is actually an article on applying polyester-based gelcoat over epoxy including some bond-strength testing they did.