I only do onion rings with the dip in milk/dip in flour method and have never thought the onions needed to be dryer. I use sweet yellow onions. I supposed if I were hard pressed and only had those really sharp white onions, I might try to do something to cut back on the sharpness. Nah. I probably just wouldn't make onion rings. Try the milk/flour method and see what you think. It is soooooooooo easy!
Okay, here's a pretty fool proof method. I've never had it fail.
First off, for slicing your onion rings, first slice the onion in half cross-wise directly down the center, then slice the rings to the thickness you want them. If you don't do this, a center section will be miserable to try to separate into rings.
For the "batter," don't mix anything. You will need one fairly deep bowl -- I use a chili bowl -- and one larger soup plate type bowl. Fill the chili bowl a bit more than half full with EITHER buttermilk or condensed milk. They both work very well. Put a comfortable amount of flour in the "soup plate", add some seasonings -- salt, pepper, season salt, onion powder, garlic powder, chili powder, thyme, oregano, whatever turns you on -- and stir with a fork to blend.
Heat your cooking oil to around 325F to 350F. If you don't have a thermostatically controlled deep fryer (I do, but I prefer a saucepan) then either use a candy thermometer or just drop a piece of torn white bread into the hot oil and when it browns in about a minute, you should be good to go.
To make the onion rings, dip rings one at a time in the milk first, then the flour, then the milk again, then the flour again, then into the hot oil. Remove and drain on paper towels when browned. If you'd like a thinner batter, similar to but not exactly like a tempura batter, then just dip in milk and flour once before frying.
This same method also produces great fried chicken, but the buttermilk has tenderizing properties that are good for chicken but not needed for potatoes. Enjoy!
Hard to give accurate info without knowing your recipe or method of processing/cooking.
Dab the onion rings with a paper towel, dip them into some corn starch, shake off the bulk of the corn starch, then dip them into your batter. You can also use something like rice flour in place of corn starch.
Don't put more batter on the rings than they'll hold easily.
NEVER salt them (or the batter) - sprinkle with salt after frying.