intinite white background

We are trying to get the 'infinite white' look with a white paper background using the HMC-150.

Our problem is seeing the yoyo string in front of an all white background. It overexposes with the white background. We have just about every color of yoyo string there is. Orange seems to be showing up the best, even better then black for some reason.

What would be the best option to get the 'infinite white' background my boss wants without making the sting disappearing/overexposing? Everything we have tried so far has made the string disappear. Moving the subject further away from the background, raising the shutter speed.

We generally have the shutter speed set at 120. I know at a higher shutter speed the camera would be better at picking up the fast speed of the string. But we have still had problem with the string getting lost at a high shutter speed when the string isn't moving.

After separating the subject from the background as soon as we use a keylight the string will disappear again.

Any suggestions would be greatly appreciated. Thanks