I recently made some fiberglass body parts using the typical Bondo brand polyester resin. I did not take into account the compatibility of the products going on afterwards. Are there any types of fillers or primers that I should stay away from (for example epoxy)?

You shouldn't have a problem with most fillers as they will say in their product sheet or even on the can itself what they can go over. If you have to "bond in" any metal attachment points on those parts I would use the same materials for bonding that you made them out of.... in other words I wouldn't switch to an epoxy resin system for that.
Other than that you should be good.

DarrelK wrote:You shouldn't have a problem with most fillers as they will say in their product sheet or even on the can itself what they can go over. If you have to "bond in" any metal attachment points on those parts I would use the same materials for bonding that you made them out of.... in other words I wouldn't switch to an epoxy resin system for that.
Other than that you should be good.

Thank you again. I will just stick with poly resin for the duration of this build.

Speaking of attaching things to the fiberglass parts I am making, can you take a look at this link when you get a chance?

http://atlantacustombaggers.com/product ... xtensions/

It appears that they have used an epoxy (panel adhesive) to bond the fiberglass extension onto the plastic saddlebag, and then fiberglassed over the seam. Is this a legit process that will hold up over time? I have grown wary of attaching glass to plastic as many people say it can't be done (or done well).

Well, if what you described is what was done in those pic.s on that bike (nice bike!) I would say it will eventually have some adhesion problems or at least some "micro cracking" down the road. I was always taught in advanced materials classes that epoxies were pretty much the answer to bonding dissimilar materials, however once you started using them in an area you should not go back to regular polyester resins or worse layer them over epoxy. About the only exceptions to that are body fillers specifically rated for applications over epoxy. Those also are usually the ones that are rated for application over SMC panels (sheet molded compound) as well.
One of the things I do with kit cars when extending panels is to actually make a steel skeleton (usually use something like electrical conduit tube or bar stock) that bonds (either type of resin system) over all the parts from the inside. I resin/mat over all that inside and add a little cloth to critical areas. I never had any cracking or parts failure when doing it in epoxy or polyester. I did ALWAYS stay within one system throughout each project though.
I've go to a lot of car shows and cruises locally and like talking to younger guys that are "learning" with ground effects kits, spoilers, etc. Most of the failures I see are do to them jumping around with different resin systems and plastics. Hope this is of some help.

DarrelK wrote:Well, if what you described is what was done in those pic.s on that bike (nice bike!) I would say it will eventually have some adhesion problems or at least some "micro cracking" down the road. I was always taught in advanced materials classes that epoxies were pretty much the answer to bonding dissimilar materials, however once you started using them in an area you should not go back to regular polyester resins or worse layer them over epoxy. About the only exceptions to that are body fillers specifically rated for applications over epoxy. Those also are usually the ones that are rated for application over SMC panels (sheet molded compound) as well.
One of the things I do with kit cars when extending panels is to actually make a steel skeleton (usually use something like electrical conduit tube or bar stock) that bonds (either type of resin system) over all the parts from the inside. I resin/mat over all that inside and add a little cloth to critical areas. I never had any cracking or parts failure when doing it in epoxy or polyester. I did ALWAYS stay within one system throughout each project though.
I've go to a lot of car shows and cruises locally and like talking to younger guys that are "learning" with ground effects kits, spoilers, etc. Most of the failures I see are do to them jumping around with different resin systems and plastics. Hope this is of some help.

It definitely is helpful, thank you.

So the project I was initially referring to was a different part on the bike, not the bags in the picture, and I will stick with poly resin for the duration of that part's modification.

The bag pics that I posted were to research for an upcoming project which I have yet to begin. If I understand correctly, I should epoxy the fiberglass panel onto the bags, and then use an epoxy fiberglass resin to glass over the seam (not the poly resin from the other project).

Yep, that's how I would do it. According to research done by West Systems Epoxy you will end up with about 20% stronger bond doing it that way with a very "compatible" composite sandwich. Another aspect that can increase bonding and longevity is to try and do "wet on wet" layup of your materials. This gives a stronger chemical bond than just a mechanical bond achieved by dry layering. With wet on wet the epoxy layers become one solid layer of material. I've made rear wings that way with epoxy in the past laying up over 1 inch foam core. With only a small diameter tube support cast in the front and back I could stand (260 pounds) on the finished wing with no buckling or distortion.

I have another question about compatibility, surprise surprise lol. When I make the part out of epoxy resin and begin to do the body work am I going to need to stay away from polyester primers and fillers? I am curious about whether using a short strand filler between the newly made parts and the primer will create a barrier..or if I need to stick to epoxy primer and urethane?

You should be okay with regular poly primers and fillers, HOWEVER, always check the p. sheets for compatibility. I know that when I am going to use Slick Sand for instance, I prefer to shoot that over an epoxy seal coat. But, that's also a case of me being somewhat "anal" about sealing anything.... Since I work with kit cars mostly the **** things are always trying to "bleed" or "off gas" something so epoxy and I have a close personal relationship. I am also a big believer in setting parts that have been created and modified in the sun for a few days. This tends to cure and bleed off any gas effects plus show possible shrinkage. Any new kit body I work on sets at least a week or two in the sun.

Ok, thanks. I am probably overthinking everything at this point so it helps to check with someone who actually knows what they're doing, lol. My plan for those bag extensions was to make them using an epoxy resin--->epoxy them to the plastic bags--->glass over the seam using epoxy resin again--->use a short strand filler, or regular body filler to complete the bodywork--->2k Urethane high build prime--->base/clear

Thanks for all of your help...if I can paypal you a 6 pack I will

Just not running head long into a bad project THEN asking us questions is thanks enough for me!