I have many bags of Wilton Candy Melts that i want to make into modelling chocolate, but whenever i try to melt them they're always so thick, they pretty much turn into a dough without the corn syrup... I may be heating them too fast, but it takes at least 5 minutes for them to barely even start to melt. The melts i have are definitely old, I've had them for a while. They do not taste bad, they just don't melt like they should. Can i add shortening to the candy melts (to thin them out) BEFORE i add any corn sryup? Or will this ruin the batch? I'm from Australia and candy melts are VERY expensive, and i don't want to throw out over $50 worth of them, or stuff them up. 

why would you want to thin out the candy? its just going to seize up as soon as you add the corn syrup...that is what it is supposed to do.

you really don't want the candy to be hot when you add the corn syrup, otherwise you might experience separation issues. melt the candy slowly, keeping it warm (not hot), until it is smooth. even if it is thick, as long as it is smooth it should be fine. add the corn syrup and stir; it will be an arm workout once it starts to seize. place glob into a large ziplock bag, roll out glob into a flat patty, then seal the bag. let it rest overnight. tear open bag and break off pieces, knead pieces until smooth and pliable, then wrap those in saran wrap. place all wrapped pieces into an airtight container for storage.

AWhen my candy melts get old they just will not get to a consistency that I can add the corn syrup. I do not know about adding shortening. Try it before you throw them out. I have begrudgingly thrown out many bags due to age.

AIn lieu of melting in the microwave, maybe try melting in a double boiler on the stove.

Here is a safe way to melt white chocolate: Make a double boiler using a large glass bowl atop a small sauce pan. Add 2/3rds of your white chocolate to the bowl. Turn on the heat to low and stir occasionally. Once the water begins simmering (through the glass bowl, you can see the steam forming), turn the heat off. Continue stirring occasionally. Once the chocolate is mostly melted, remove it from the heat and add the remainder of the un-melted chocolate. Continue stirring until it's all fully melted.

Note that candy melts do get stale and less cooperative over time so you might have better luck with fresh chocolate.

A

the candy is so so thick when I attempt to melt it. I'm talking peanut butter thick.

overheating can ruin even the most expensive melts/chocolate.  Low & slow & tempered with unmelted bits.

@WickedGoodies is the master craftsperson with this, and her description above is perfect.