cracked resin

Could you post a picture? That would make it easier to give you good advice.

 

This is definitely not the prettiest solution, but I've used JB PlasticWeld to repair a massive crack down the center of my Fifth Motif Venitu's chest after he fell in an earthquake. It's held up great for almost two years now with him being fully strung, and the crack hasn't gotten any worse since.

Aesthetically it's not the best solution, but I can vouch for it holding up well. The material is a bit rubbery when fully dry and I wouldn't personally try to paint over it. But I believe that property is what's made it structurally reliable despite the massive amount of stress being put on the chest piece. If it dried super rigid, I know it would have cracked again after stringing him up.

 

How you are going to go about fixing a crack depends on the crack and what caused it.

If if was outside force (a fall, a shipping accident etc) you'll have an easier time than if it's a badly constructed joint to begin with. The later is uncommon, but it does happen and since no repair is going to be as strong as the undamaged polyurethane the ckrack is likely to open up again without reinforcement or correction of the joint itself.
In the former case, however, a simple glue repair will probably do. Either two part epoxy glue or cyanoacrylate is usually used. If the piece has broken off completely you can drill into the surface to insert reinforcing pins, but the depends on the damage and is not always possible and not always neccesary. Pictures really would help.
If the piece has not broken off, the watery type (as opposed to the gel type) of cyanoacrylate is usually the best way to go since it can be worked into the crack using capailarry force.
If the surfaces of the break are distorted you can try the boil method to make them match up.
A reinforcing layer of epoxy putty is also and option in some cases.

 

You can't upload photos to DoA.
What you do is that you upload it to the host of you choice. When you want to imbed the picture into a post you simply click the image tool and copy+paste the URL into the box that pops up.
Make sure the image is of an appropriate size and file type for forum use. A good rule of tumb is that anything taller than 600 pixels and wider than 800 pixels will likely require scrolling to see the whole thing. That or less and the whole thing will be visible at once on most computer screens. This also keeps the file size down and reduce the risk of your images not loading properly.