adding sugars

Regular old table sugar added during the boil has always worked fine for me to thin/dry the beer. For English or Belgian style beers, you can use the darker sugars and syrups. Or, just table sugar. It's all good.

 

I use corn sugar at the beginning of the boil

 

When to add? At the end of the boil giving it enough time to fully dissolve and incorporate (2-5 minutes depending on system). You CAN add it at any time in the boil, with the understanding that earlier sugar addition affect hop utilization. That is why most call for sugar to be added at the end of the boil.
What kind? It depends on your goal. If it is to lower FG without contributing to flavor, the cheapest plain old table you can find. Corn sugar works also, but it is a price distinction without a flavor difference. (Side note: If your recipe calls for Belgian sugar that has a flavor component, obviously use that. If it calls for clear Belgian candy sugar, use table sugar.)

 

Agree with @scurvy311 's comments. Corn sugar and cane sugar are virtually identical (chemically speaking), only differ in density and appearance, both work well. For an IIPA you're not after color or flavor, so go cheapest. For this style, up to 10% of grain bill will work, monks take it up to 20%.

I caution about underpitching your yeast with sugar additions. Those little buggers will consume 100% of the simple sugar and there is a possibility they "fill up" on desert before they finish the entré (complex sugars). So I suggest using a yeast calculator and nailing the pitch-rate (which is always the goal). Another technique is to add sugar after high-krausen . . . the yeasties are sure to have gorged on the main course before they can develop a sweet tooth. You also know your OG at this point and can re-compute the sugar needed to hit your ABV. This also reduces blow off.

 

At the risk of being presumptuous, if you are asking this question, I'm assuming that you are new to this hobby. Welcome.

If you are new, I just want to be sure that you are not mixing up 'adding sugar' as part of the recipe vs. using your priming sugar that may have come with your kit (if it is a kit), which is used only at the time of bottling.

 

Mothergoose might be onto something. If you haven't brewed much before, be sure you save your 4 oz or whatever "priming sugar" for bottling day. This is the "dessert" for the yeast so that they'll carbonate the beer inside the bottles. If the recipe actually contains sugar in the boil, then my previous comment applies.

 

Good point. Additionally, if you are trying to dry out a recipe, you swap sugar for extract, you don't add sugar on top of the extract.