Ok so here is my experience with polyester and epoxy resins as i was taught just to add even more confusion to the discussion.
I worked for a company for 15 years that developed and manufactured polyesters , epoxies and urethanes etc...for almost 50 years. The owner was a chemist in the 1950's (Norry Hastings) that helped develop some of the original formulas for many of the resins. he worked with the govornment, military and business sector. we provided material for the original stealth bombers and various other programs. was a cool place to
work he was genuinely a brilliant man.
When i worked for the company i was responsible for manufacturing (actually mixing and making the resins from scratch), selling and working with all of the resins we
sold to the public...i will admit i have limited experience fiberglassing
boats specifically but i have worked extensively with epoxy, polyesters and urethanes.
this is the basics of what i was taught and what i learned from accumulated knowledge using the resins. take it with a grain of
salt as my knowledge ends in the mid 2000's but the
core ingredients dont seem to have changed much. i am a bit out of touch with new resins, cloths and techniques though but i think some of what i learned is still relevant.
not all resins are created equal and knowledge of a particular resin, technique and skill using them can make or break the properties of a good resin or enhance a bad one. a good quality resin from a reputable supplier and a fresh resin are ideal.
as far as using polyester over old polyester...we were taught and my experience has proven that if its properly prepped it a perfectly good way to repair old polyester. there are differences in polyester resins though. for example laminating polyesters do not have a wax additive and were always used only for the
layup with cloth or matt no matter how many layers. the reason was simply there was no additive to cause a bond failure. we were taught that only the last and final layer would you use a polyester with a wax additive if you used one at all. this will prevent
delamination when the heat of the sun starts to expand and
contract the resin on a daily basis. technique was king though as resin rich laminates or poor or sloppy prep was counter productive.
as far as polyester and
wood...was not a good combination. polyester over any tropical
wood or any naturally oily wood is a very bad mix from our experience as the natural oils in the wood act as a release agent. even have to be carefull using epoxy over very oily tropical woods. i have seen some nasty delaminations where polyester was used over tropical woods two or three years later after repeated expansion and contractions. the oilier the wood, the faster the
delamination. if we were going to use polyester over any wood we would prep it by adding styrene monomer to polyester at 10% volume or less, less is better (makes it watery thin) and soaking the wood with this mix and letting it mostly cure. do not use acetone for this as it can cause problems later. once mostly cured but still tacky we began glass
layup and you would get the best bond you could get between the two.
epoxies were always the choice for bonding and structural applications. we worked with many structural repair guys and casting epoxies or medium to slow cure epoxies (non filled) were the the preferred choice. reason being their bond to pretty much any surface and their strength. a properly mixed and cured slow to medium set epoxy is god damn tough. epoxies tend to remain soft and flexible (not always perceivable) and its this resilience that helps with their strength and adhesion properties. polyesters on the other hand in comparison are always brittle. i say slow to medium pot life epoxies as opposed to fast set because the heat generated in curing can make even laminating epoxies more brittle.
all that being said if you already used epoxy you really do want to finish with it because polyester may appear to bond to epoxy...and it will for a time...over time though expansion and contraction forces will cause delamination between the two, i have seen a lot of it. may not be a year or two but it will eventually happen and our boats are in the elements 24/7. not a choice i would make. when laminating with epoxy we always did as much of the lamination as possible, at once. if we did have to stop and let the epoxy lamination set and partially cure, we always prepped it by
cleaning with solvent and or lightly
sanding it before continuing. epoxies have a natural
oil that settles on the surface during cure and can actually act as a release to itself so we always had to be carefull. casting epoxies were the worst for this as opposed to potting, tooling or laminating epoxies.
having said all this, if your still reading, personally i would do most all repairs of polyester with polyester with the exception of bonding to wood, filling voids or areas where higher structural strength is desired. i want to re-glass some of my bulkhead tabbing soon and will do it with a good quality laminating epoxy as it will bond over the original wood and polyester already there.
hope some of this was helpfull to someone.
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